The behaviour of sperm from egg penetration until creation of the zygote, the development of the maternal pronucleus, and the two first cleavage divisions were studied by use of fluorescence microscopy. It was found that 4-12 sperm penetrate the egg membranes prior to oviposition. Contrary to previous reports, we found that only 1-7 sperm move from their initial location just beneath the vitelline membrane and into the cytoplasm, where they develop into paternal pronuclei. At the time of oviposition, the oocyte nucleus was usually at the stage of metaphase I, rather than anaphase I as previously reported. At 26+/-2.5 minutes the meiotic process had entered the stage of metaphase II. The paternal and maternal pronuclei formed at 55+/-2.6 minutes, and they fused at 93+/-7.3 minutes. The mitotic division of the zygote was completed at 119+/-6.5 minutes.