This study examines the effects of actin microfilament-disrupting drugs on events of fertilization, with emphasis on gamete membrane interactions. Mouse eggs, freed of their zonae pellucidae, were treated with drugs that perturb the actin cytoskeleton by different mechanisms (cytochalasin B, cytochalasin D, jasplakinolide, latrunculin B) and then inseminated. Cytochalasin B, jasplakinolide, and latrunculin B treatments resulted in a decrease in the percentage of eggs fertilized and the average number of sperm fused per egg. However, cytochalasin D treatment resulted in an increase in the average number of sperm fused per egg and the percentage of polyspermic eggs. This increase in polyspermy occurred despite the observation that cytochalasin D treatment caused a decrease in sperm-egg binding and did not affect spontaneous acrosome reactions or sperm motility. This suggested that cytochalasin D-treated eggs had an impaired ability to establish a block to polyspermy at the level of the plasma membrane. The effect of cytochalasin D on the block to polyspermy was not due to a general disruption of egg activation because sperm-induced calcium oscillations and cortical granule exocytosis were similar in cytochalasin D-treated and control eggs. However, buffering of intracellular calcium levels with the calcium chelator BAPTA-AM resulted in an increase in polyspermy. Together, these data suggest that a postfertilization decrease in egg membrane receptivity to sperm requires functions of the egg actin cytoskeleton that are disrupted by cytochalasin D. Furthermore, egg activation-associated increased intracellular calcium levels are necessary but not sufficient to affect postfertilization membrane dynamics that contribute to a membrane block to polyspermy.