Two earlier papers (Wilson, 1968 a, b) have described the rearing of Sabellaria alveolata (L.) and the settlement behaviour of the larvae. It was thought that some of the questions remaining unanswered might be resolved for the cost of one more rearing to obtain the crawling stages needed. With this end in view adult worms were collected at Duckpool on 12 July 1968 during the lowest day-time spring tide of that month. The worms were found to be spawning and had already shed about half of their genital products. On returning to the laboratory a fertilization was made immediately, using a mixture of ova from many females and a mixture of sperms from several males. This fertilization, made at 8 p.m. a few hours after leaving the shore, is the basis of most of the work described in this paper.