Laboratory and field investigations show that there are five distinct marsupial stages and eight post-marsupial stages in the development of the isopod Dynamene bidentata (Adams). Extensive sexual dimorphism is apparent, beginning at stage 6, and females undergo extensive tissue reduction at the moult to the eighth stage. Young stages inhabit and feed on intertidal algae, and the non-feeding adults (stage 7 and 8 females, and stage 8 males) shelter in crevices or empty barnacle tests. Average brood numbers are fairly high (c. 90) and the average marsupial mortality rate is about 36%. Release of young takes place in May/June, after which the females die, and a resident population of males remains in the crevices to be joined by newly maturing females and males from August onwards each year. The occurrence of two breeding seasons in males, but not in females, explains how the sex ratio of males to females as they leave the algal habitat (1:9) is about half the ratio of males to females in the reproductive habitat (1:4).