Elemental selenium, Se(0), is a prevalent chemical form in sediments, but little is known about its bioavailability. We evaluated the bioavailability of two forms of Se(0) by generating radioisotopic ⁷⁵Se(0) through bacterial dissimilatory reduction of ⁷⁵SeO₃^2- by pure bacterial cultures (SES) and by an anaerobic sediment microbial consortium (SED). A third form was generated by reducing ⁷⁵SeO₃^2- with ascorbic acid (AA). Speciation determinations showed that AA and SES were >90% Se(0), but SED showed a mixture of Se(0), selenoanions, and a residual fraction. Pulse-chase techniques were used to measure assimilation efficiencies (AE) of these particulate Se forms by the bivalve Potamocorbula amurensis. Mean AE values were 3 ± 2% for AA, 7 ± 1% for SES, and 28 ± 15% for SED, showing that the bioavailability of reduced, particle-associated Se is dependent upon its origin. To determine if oxidative microbial processes increased Se transfer, SES ⁷⁵Se(0) was incubated with an aerobic sediment microbial consortium. After 113 d of incubation, 36% of SES Se(0) was oxidized to SeO₃^2-. Assimilation of total particulate Se was unaffected however (mean AE = 5.5%). The mean AE from the diatom Phaeodactylum tricornutum was 58 ± 8%, verifying the importance of Se associated with biogenic particles. Speciation and AE results from SED suggest that selenoanion reduction in wetlands and estuaries produces biologically available reduced selenium.