The size-frequency distributions of monthly collections of Aplysia punctata from a sublittoral population in Trearddur Bay, Anglesey, made during the period from July 1964 to October 1965, indicate that the animals live for one year and that the major recruitment to the population occurs in the autumn. The Aplysia in this area are restricted mainly to sublittoral beds of the red alga, Plocamium coccineum, and the majority of animals feed, copulate and deposit spawn on this seaweed. The animals spawn from May, when the sea temperature reaches a minimum of about 9–9·5° C, until about October, with a maximum output of spawn in the late summer when the animals are fully grown. Aplysia in Trearddur Bay do not migrate inshore; instead, they probably colonize the intertidal region after being accidentally washed inshore from the sublittoral habitat. Intertidal and shallow-water Aplysia are larger than those from deeper water possibly because of the greater abundance of food. Similarly, it is thought that Aplysia found in the intertidal zone are brown and olive-green because they eat a particular alga or combination of algae present in this habitat.