A population of the two species, Nephtys hombergii Savigny and N. caeca Fabricius, has been studied for a period of 3 years. Three separate but related aspects of the biology of these species have been investigated; gametogenesis (Olive, 1976a, and in preparation), reproductive endocrinology (Olive, 1976 a, b) and the life-history and population structure which is the subject of the present report. Nephtys species are particularly interesting for studies of this kind since Kirkegaard (1970) described rings in the teeth of N. hombergii which were thought to represent annual growth rings. Estcourt (1975) has found that these rings can be used to age the nephtyid species Aglaophamus verrilli in New Zealand which is a slow growing species living to at least 5 years of age. The degree of overlap between the successive generations, as indicated by the rings in the teeth, emphasizes how difficult age analysis of populations with this type of growth is without additional information of this kind. This problem is particularly acute in long lived polytelic (= iteroparous, Gadgil & Bossert, 1970) species. Olive (1975) has discussed the problem with reference to Eulalia viridis and other recently investigated polychaetes but it is not peculiar to Polychaeta as, for instance, Lewis & Bowman (1975) have shown in a particularly detailed study of populations of Patella vulgata.