Understanding the reproductive biology of the endangered Epioblasma capsaeformis of the eastern United States is critical to conservation efforts at mussel hatcheries. We studied how males influenced gravidity among females held in captivity. Percent males (0%, 33%, 50%, and 67%) within a holding system was used as the predictor variable. Our response variables were percent females observed gravid, number of eggs and glochidia per gravid female, total eggs (sum of eggs and glochidia) per gravid female, and proportion of total eggs successfully fertilized and developed into glochidia. Mean percent of females gravid in the male treatments were 73%, 85%, 69%, and 60%, respectively, with no evidence that treatments differed significantly from one another. However, the treatment without males had significantly lower mean number of total eggs observed (4,533 vs. 5,868 to 7,330), with fewer viable glochidia (1,354 vs. 5,645 to 6,920). Most of the eggs in the treatment without males were unfertilized at experiment completion (3,179 vs. 206 to 410), with a much lower percentage of transformed glochidia (27% vs. 94 to 97%). Our study documents the important role that males play in fertilizing females for production of glochidia and that key reproductive processes occurred in captivity.