Sen. Carl Glimm said Montana’s law needs definitions for male and female, and Gov. Greg Gianforte signed Senate Bill 437 on Tuesday, nearly a year after it cleared the Legislature, the Associated Press reported. The measure formally amends Montana statutes to treat “sex” as a binary category tied to a person’s reproductive system, and it carries those definitions into a wide range of state rules that use terms such as sex, gender, male, female, man, woman, father and mother.
The change arrives after earlier versions of a similar sex-definition law were struck down by courts—first in June 2024 over the clarity of the subject in the title, and later in February 2025 after a judge found the law violated the equal protections clause of Montana’s constitution. Supporters of the new bill argue it is meant to cement consistent categories in state law, while opponents say it pushes people to disclose private information and forces identity markers in state documents to conflict with how they live.
Senate Bill 437 defines sex as whether someone is male or female, distinguished by reproductive system characteristics. Under the bill, lawmakers define a female as an individual who “naturally has, had, will have or would have but for a congenital anomaly or intentional or unintentional disruption” a reproductive system that uses ova for fertilization. The law defines a male as someone with a reproductive system that uses sperm for fertilization.
The bill also removes references to sex markers in chromosomes that were included in a 2023 version of the law, according to the reporting. In addition, it adds definitions for man, woman, father and mother that are built off the male and female definitions.
The legislation further states that in Montana law the term “gender” must be considered a synonym for sex as defined in the new legislation, and it may not be considered synonymous with a person’s gender identity, experienced gender, gender expression or gender role. It also says the term “gender identity,” if used in state law or rules, may not be treated as a substitute for sex or gender.
Supporters said the bill is aimed at avoiding what they describe as shifting or subjective categories. Glimm, the bill’s sponsor, told Montana Free Press that definitions are needed for clarity in state law. “In our culture, it’s gotten to where that is not clear,” he said. “It becomes a fuzzy line for some, so we just need to have clear definitions in law so that it’s clear what we’re talking about when we talk about a male or female.” The reporting also said Glimm compared the approach to federal guidance issued by the U.S. Department of Health and Human Services in February 2025.
The reporting said the definitions apply broadly: they would be used anywhere in Montana law that mentions sex, gender, male, female, man, woman, father or mother, including more than 60 sections governing topics ranging from driver’s licenses and marriage licenses to the state’s anti-discrimination protections. Those laws can intersect with workforce and labor disputes, and the bill also reaches less obvious areas such as laws governing representation of men and women on certain state boards and commissions and on local precinct committees.
Opponents say the practical effects extend beyond what they view as unnecessary policy. In comments reported by Montana Free Press, Missoula Democrat Rep. Zooey Zephyr, a transgender woman, said the law discriminates against “every aspect” of trans people’s lives and that it erases intersex, nonbinary and Two Spirit people “from cradle to grave,” adding that plaintiffs in the earlier lawsuit said a requirement to present a driver’s license sex marker that doesn’t match someone’s appearance could create risks in settings such as housing applications or being carded at a bar.
The 2023 lawsuit challenges a similar sex-definition law, and the plaintiffs’ attorneys said they plan to move to include Senate Bill 437 in court. The Helena-based nonprofit Upper Seven Law, which represented plaintiffs in the 2023 case, notified a Missoula state District Court that it will seek permission to file a supplemental complaint covering the new law. Rylee Sommers-Flanagan, Upper Seven Law’s executive director, said the request and supplemental complaint would be filed on or before April 6.
Sommers-Flanagan said she expects Upper Seven would ask for a preliminary injunction or temporary restraining order to halt SB 437 while the case proceeds. She also characterized the effort as a second attempt at legislation Montana courts already determined violated the state constitution. “This is a second attempt at almost exactly the same thing Montana courts already determined violates the state constitution,” Sommers-Flanagan said. “It’s a disappointing use of state resources.”
Both Glimm and Zephyr said they expect the law to be challenged in court. Supporters of the bill included the Montana Family Foundation, whose representative Derek Oestreicher said the definitions were needed for consistency across legal and medical documents and government records.
The reporting also described why the bill took time to reach Gianforte after the Legislature passed it in mid-April 2025. The bill’s legislative paper trail shows Senate President Matt Regier signed the measure within days of its final vote, but House Speaker Brandon Ler did not transmit it promptly, and a spokesperson for House Republicans in August described an intentional delay to prevent the bill from becoming immediately tied up with ongoing litigation over the similar measure passed the prior session. Ler signed SB 437 on March 24, 2026, it was transmitted to Gianforte’s office and the governor signed it the same day, the reporting said.
Todd Everts, the director of legal services for the Legislature, told Montana Free Press that the joint rules only describe steps a bill “may” take after passage and that when a bill proceeds is discretionary. Looking ahead, Sommers-Flanagan said adding SB 437 to the active case could be more efficient for Upper Seven’s litigation. Zephyr said the next legislative session’s path could depend on the outcome of this year’s elections, with 125 seats up for election this fall.