Native freshwater mussels can be pivotal members of aquatic ecosystems, but their numbers are declining and many basic information gaps exist. Biomass is a critical indicator of a species’ status, and can be calculated from a relationship between an individual’s length and mass to more easily estimate the population biomass of a species. Lethal methods are typically used to create length-mass regressions, but sacrificing individuals of a rare, declining species is undesirable. We used a non-lethal method to create length-mass regressions for the California Floater (Anodonta californiensis/nuttalliana) and Western Pearlshell (Margaritifera falcata) based on shell and length and wet mass from individuals in the Bear River, Wyoming, USA. We related shell length to the dry mass of shells and soft tissue. Modeled biomass did not differ from measured biomass for Western Pearlshell or California Floater (glm; p > 0.33). Biomass is a critical measure to assess populations and our method allows the development of length-mass equations using non-lethal methods for rare mussels of management concern.