AbstractFemales of many animal species store sperm for taxon-specific periods of time, ranging from a few hours to years. Female sperm storage has important reproductive and evolutionary consequences, yet relatively little is known of its molecular basis. Here, we report the isolation of a loss-of-function mutation of the Drosophila melanogaster Acp29AB gene, which encodes a seminal fluid protein that is transferred from males to females during mating. Using this mutant, we show that Acp29AB is required for the normal maintenance of sperm in storage. Consistent with this role, Acp29AB localizes to female sperm storage organs following mating, although it does not appear to associate tightly with sperm. Acp29AB is a predicted lectin, suggesting that sugar–protein interactions may be important for D. melanogaster sperm storage, much as they are in many mammals. Previous association studies have found an effect of Acp29AB genotype on a male's sperm competitive ability; our findings suggest that effects on sperm storage may underlie these differences in sperm competition. Moreover, Acp29AB's effects on sperm storage and sperm competition may explain previously documented evidence for positive selection on the Acp29AB locus.