Seasonal differences in glochidial maturity, substrate, and diet were studied to determine how these factors affect the survival and growth of juvenile freshwater mussels. Comparisons were made between juveniles produced in the fall and spring of the year; cultured in sediment, sand, or without substrate; and fed either of two species of small (5–10-mm) green algae. The survival and growth of endangered juveniles of oyster mussel Epioblasma capsaeformis were compared with those of a common, seemingly more robust species, the rainbow mussel Villosa iris. The growth of rainbow mussel juveniles was significantly greater than that of oyster mussel juveniles (P , 0.001). The survival and growth of oyster mussel juveniles were significantly greater when propagated in the spring, that is, when glochidia were mature and would normally be released, than in fall (P , 0.001). Survival and growth of juveniles of both species were significantly greater when they were cultured in a sediment substratum rather than sand or no substratum (P , 0.001). No differences (P . 0.05) were observed in survival and growth of juveniles fed algal species Neochloris oleoabundans or Nannochloropsis oculata. In the spring of the year, juvenile oyster mussels achieved a survival of 29.6% and mean length of 664 micrometers at 60 d of age, whereas at the same age rainbow mussel juveniles exhibited a survival of 25.1% and a mean length of 1,447 micrometers.