Scanning and transmission electron microscopy were used to determine themorphological changes in the egg plasma membrane associated with spermbinding, fusion and incorporation in Xenopus laevis. Sperm incorporationin Xenopus is rapid, occurring within 3-5 min following addition ofsperm. Images have been obtained of both early sperm-egg interactionsand fertilisation bodies. Additionally, two drugs that specificallyalter F-actin dynamics, latrunculin and jasplakinolide, were used todetermine whether sperm incorporation is a microfilament-dependentprocess. Jasplakinolide did not prevent sperm incorporation, corticalgranule exocytosis or cortical contraction, suggesting these eventscan occur without depolymerisation of existing, stabilisedfilaments. Latrunculin A, which competes with thymosin β4 in ooplasm forbinding actin monomer, did not inhibit cortical granule exocytosis, butblocked cortical contraction in 100% of eggs at a concentration of5 μM. Although a single penetrating sperm was found on an egg pretreatedin latrunculin, fertilisation bodies were never observed. At <5 μMlatrunculin, many eggs did undergo cortical contraction with someexhibiting severe distortions of the plasma membrane and abnormalaccumulations of pigment granules. Preincubation of eggs in jasplakinolidebefore latrunculin mitigated both these effects to some degree. However,eggs incubated in latrunculin either prior to or after insemination neverprogressed through first cleavage.