We examined the ultrastuctural events occurring during early fertilization in the giant scallop,Placopecten magellanicus.The unfertilized oocytes had not completed their meiotic maturation and were blocked at the metaphase I stage. At fertilization, there was a local loss of microvilli, and a fertilization cone of 6 μm in length by 4 μm in diameter was produced. As the spermatozoon penetrated the oocyte, its head rotated by 90 to 180°. The condensed electron-opaque state of the sperm chromatin transformed into a dispersed form of male pronucleus; this event was coupled with the beginning of the formation of the pronuclear envelope and the development of the sperm aster. The male pronucleus was completely formed 60 min postfertilization, before completion of meiosis, and spots of electron-dense chromatin still remained. During sperm nuclear reorganization, meiotic maturation was resumed. At anaphase I, 30 min after fertilization, the dyad chromosomes moved into a cytoplasmic protrusion that became the first polar body. The chromatin located within this protrusion was condensed in a compact electron-dense mass. A gap of 0.2 μm separated the first polar body from the zygote. At anaphase II, 60 min after fertilization, a second protrusion located just below the first polar body developed into the second polar body. Unlike the first polar body, the second polar body had a flat shape, remained closely associated with the oocyte surface, and the chromatin located within it dispersed. Nearly 90 min after fertilization, the pronuclei completed their migration toward each other and their pronuclear envelopes became into close apposition without interdigitating. At this stage, there was a simultaneous condensation of the chromosomes and a breakdown of the pronuclear envelopes of both pronuclei. Intact sperm mitochondria still persisted 90 min after fertilization. The chromosomes attached to microtubules and became aligned on the metaphase plate of the first mitotic spindle. Hence, for this species, no zygotic nucleus is formed. This type of pronuclear association is similar to theAscaris-type of fertilization.