The male sterile mutation, misfire (mfr), of Drosophila melanogaster is a novel paternal effect, fertilization defective mutant that effects sperm head decondensation. mfr sperm were motile, appeared normal morphologically and were transferred to the female during copulation. However, less than 0.1% of eggs laid by females mated to mfr males hatched. Although mfr sperm entered eggs at a high frequency (93%), 99% of the inseminated eggs did not initiate the first nuclear division. Unlike wild type fertilizing sperm, the position and shape of mfr sperm tails within the egg were not constant, but varied in a seemingly random manner. The heads of inseminating mutant sperm were always located near the surface of eggs just underlying the egg plasma membrane, and maintained their needle-like shape indicating the failure of nuclear decondensation. Further observations revealed that plasma membrane of inseminating sperm appeared intact, including the head region. These phenotypes were equivalent to those of sneaky (snky), another fertilization defective male sterile mutation. Our observations strongly suggest that mfr mutant males are sterile because their inseminating sperm fail to form a male pronucleus due to the inability of the sperm to properly respond to egg factors responsible for the breakdown of the plasma membrane. Although mfr and snky mutations were phenotypically identical, they mapped to cytologically distinct genetic loci and no genetic interactions were observed, suggesting that at least two distinct paternal gene products are involved in the early stages of pronuclear formation.