The mechanism of sperm tail incorporation and the fate of the tail during mouse fertilization and early embryogenesis were examined. Time-lapse video microscopy and anti-tubulin immunofluorescence show that the incorporation of the sperm tail, but not the sperm head, is sensitive to cytochalasin B (a microfilament inhibitor). Colcemid, a microtubule inhibitor, does not affect tail incorporation. High-resolution, low-voltage scanning electron microscopy demonstrates that the plasma membrane covering the sperm tail does not appear to fuse with the oocyte membrane during in vitro fertilization in the presence of cytochalasin. In control and colcemid-treated oocytes, the plasma membrane along the sperm tail, which is oriented tangential to the egg surfaces, appears to fuse with the oocyte membrane at multiple sites. An antibody to testicular alpha-tubulin detects sperm-derived, but not egg, microtubules and this has permitted us to trace the behavior and disappearance of the sperm tail during embryogenesis. Conventional and confocal microscopy show that following sperm incorporation, the tail often splays into multiple fibers. At the two-cell stage, the axoneme may be localized in either blastomere or it may be found to run through the midbody between both blastomeres. The tail appears to shorten by the 8-cell stage and is undetectable after the 16-32 cell stage. In morulae, tail fragments have been found in outer cells but not in inner ones, and fragments have not be found in blastocysts. These data suggest that microtubules of sperm and oocytes contain different isotypes of alpha-tubulin, nongenomic sperm-derived components survive at least to the morula stage of mouse development, and egg microfilaments are involved in the incorporation of the sperm tail but not the sperm head, which demonstrates that motility during sperm incorporation is different in mammals when compared to lower vertebrates and invertebrates.