Population dynamics of the polychaetes Ceratonereis keiskama and C. erythraeensis were studied at the Berg River estuary, South Africa, from December 1987 to April 1989. There was marked size-related depth stratification of both species, with small worms being concentrated in the upper layer of the substratum and larger ones deeper down. Reproduction of both species occurred in summer. Three cohorts were distinguished in both populations. Recruitment of C. keiskama peaked in December whereas that of C. erythraeensis varied between years and sites (December-April). The population biomass of C. keiskama peaked in midsummer and was lowest during the spring and winter. C. erythraeensis maintained a high population biomass during winter and reached its lowest biomass during January-February. The total annual production of C. keiskama in the restricted area of the estuary where it occurred was 7,58 g m−2 y−1, with a mean annual biomass of 4,11 g m−2 making P/B = 1,84. Total annual production of C. erythraeensis for the whole estuary was 14,42 g m−2 y−1, mean annual biomass was 7,59 g m−2, and P/B = 1,90
