The recent risk assessment by the North-Cotentin Radioecology Group (http://www.irsn.org/nord-cotentin/, 1999) outlined that 14C has become one of the major sources of the low dose to man through seafood consumption. It was recommended that more data should be collected about 14C in the local marine environment. The present study aims to respond to this recommendation. The estimation of 14C activity in marine species is based on concentration factor values. The values reported here ranged from 1 × 103 to 5 × 103 Bq kg-1 ww/Bq L-1. A comparison was made between the observed and predicted values. The accuracy of 14C activity calculations was estimated between underestimation by a factor of 2 and over-estimation by 50% (95% confidence interval). However, the use of the concentration factor parameter is based on the biological and seawater compartments being in steady state. This assumption may not be met at short distances from the point of release of discharges, where rapid changes in seawater concentration may be smoothed out in living organisms due to transfer kinetics. The data processing technique, previously published by Fiévet and Plet (2003. Estimating biological half-lives of radionuclides in marine compartments from environmental time-series measurements. Journal of Environmental Radioactivity 65, 91-107), was used to deal with 14C transfer kinetics, and carbon half-lives between seawater and a few biological compartments were thus estimated.