The coelomic gregarine Diplauxis hatti exhibits a unique adaptation of its life cycle to its polychaete host Perinereis cultrifera. Experimental and ultrastructural observations on natural populations from the English Channel showed that release of parasite spores is concomitant with the polychaete spawning. As the development of P. cultrifera is direct, the notochete larva ingest parts of the jelly coat covered with numerous sporocysts of D. hatti during hatching. Transepithelial migration of the sporozoites takes place in the gut of three- or four-segment notochete larvae and syzygies of about 20 microm are observed in the coelom. Growth of these young syzygies is slow: after 18-24 mo they reach only 60-70 microm. They exhibit active pendular movements. In the English Channel, female and male gametogenesis of P. cultrifera begins at 19 mo and 2 yr, respectively; the somatic transformations (epitoky) in the last 4 mo of their 3-year life. During epitoky, the syzygies undergo an impressive growth and reach 700-800 microm within a few weeks. A shift from pendular to active peristaltic motility is observed when the syzygies reach 200-250 microm. When gamogony occurs, syncytial nuclear divisions are initiated and cellularization produces hundred to thousands of male and female gametes of similar size. The male gametes exhibit a flagellum with 3+0 axoneme. The mixing of the gametes ("danse des gametes") and fertilization are observed during 4-5 h. Zygotes differentiate sporoblasts with eight sporozoites. The sporozoites exhibit the canonical structure of Apicomplexa, a polarized cell with micronemes and rhoptries.