The germ cells of Polydora ciliata arise from cells lying just inside the ventral epithelium and later migrate into the gonads. They are released into the coelom when they are 30 μ in diameter and are transported posteriorly to the region of maturation. The eggs and sperm are shed through the modified nephridia of the region of maturation. The eggs are laid into a string of capsules apparently secreted by the nephridia and attached to the wall of the tube. The larvae take about a week to hatch and are released into the plankton at the three chaetiger stage. They spend some 6 weeks in the plankton before metamorphosis, and have grown to 17 or 18 segments. Monthly samples of the worms living in the London clay at Whitstable in 1956 showed that the population was bimodal with respect to size and the two groups of adults gave rise to two larval settlements in late March and May. The two groups of juvenile worms that developed from these larvae retained their identities through the summer and the following winter. There is no apparent morphological character by which the adults of each type may be recognized, the time of spawning and the lower fecundity of the early breeding worms being the only distinguishing features. It is suggested that these groups are the consequence of an overlap of two populations of P. ciliata from different centres of distribution, each being adapted in its breeding requirements to a different part of the temperature range.