A benthic survey was carried out from November 1998 to December 1999 in the tidal flats of Bahía Samborombón (Río de la Plata estuary, Argentina), in order to study the population structure, growth and secondary production of Laeonereis culveri. The samples were collected monthly in two intertidal areas with different environmental characteristics (San Clemente Creek and Punta Rasa). Growth was analysed using computer-based methods of length–frequency data (ELEFAN), and the secondary production was estimated by Crisp's and Hynes & Coleman's methods. Laeonereis culveri were recruited throughout the year, with two main peaks during spring and autumn. The autumn cohort showed growth rate (K) of 2 in San Clemente Creek and 1.8 in Punta Rasa. The seasonal oscillation of growth pointed out that worms grow very slowly during winter months. The life span of this cohort ranged from 15 to 17 months. The spring cohort showed higher growth rates in both sampled areas (K=3.3 in Punta Rasa and 3.1 in San Clemente Creek), whereas the growth oscillation showed similar values to those of the autumn cohort. The lowest growth rate was found in January and the life span was 9.5 months. The annual mean biomass in San Clemente Creek was 5.44 g m−2, with a mean production of 40.8 and 43.8 g m−2 y−1, according to the method used, and a production/biomass (P/B) ratio of 7.5 and 8 y−1 respectively. In Punta Rasa, the annual mean biomass (2.69 g m−2) and mean secondary production (19.44 and 23.61 g m−2 y−1, according to the method used) were lower than in San Clemente Creek, nevertheless the P/B ratio (7.2 and 8.7 y−1) was similar. The high secondary production and P/B ratio observed suggest that L. culveri transfer an important biomass to higher trophic levels, pointing out the very important role that this polychaete plays in the energy flux of this coastal estuarine ecosystem.