A detailed 24 month study (1968–1970) of the breeding cycles and population structures of the four British members of the Jaera albifrons Leach group of species (Isopoda: Asellota) has been carried out in Milford Haven, Pembrokeshire. The species often show overlapping ecological distributions and they can only be identified on male secondary sexual characters. Sampling stations had therefore to be selected carefully to ensure that single species populations were studied. In this way virtually single-species populations of Jaera forsmani Bocquet, Jaera praehirsuta Forsman, and Jaera ischiosetosa Forsman have been studied but the Jaera albifrons Leach population was somewhat mixed with J. ischiosetosa, particularly when the stream normally inhabited by the last species dried up in the summer. Only one hybrid was found (between J. albifrons and J. ischiosetosa) in a total of 6214 specimens collected. Gravid females were taken every month for each species with peaks of breeding occurring during spring and summer. Young were liberated throughout the year by J. albifrons and J. ischiosetosa, but with summer peaks. J. praehirsuta and J. forsmani had a particularly limited summer period for the release of juveniles. J. forsmani was consistently the largest species and has a restricted geographical distribution. J. praehirsuta also has a patchy distribution. The sex ratio of males to females was never 1:1 for any species. Females outnumbered males by up to 14:1, with the sex ratio varying throughout the year except in J. praehirsuta where it remained at about 1.5:1.