Atrazine and diuron are two herbicides present in sediments of the intertidal mudflats in the Pertuis Charentais (Atlantic coast, France). They may negatively affect photosynthesis processes in microphytobenthos, the most productive primary producer in these environments. The impact of these herbicides on benthic diatoms was investigated using PAM fluorimetry on natural microalgal communities, once extracted from sediment, by following the variations of the effective quantum yield of PSII (Phi(PSII)) during exposure to atrazine and diuron concentrations in a range 0.1 mu g L-1 (.) 10 mg L-1. No significant effect was noticed for 0.1 mu g L-1 concentrations, but progressive reduction in photosynthetic efficiency, always occurring within 15 min, was associated with increasing herbicide concentration. Diuron appeared to be more toxic than atrazine as shown by the complete inhibition of the photosynthetic processes at concentrations of 100 mu g L-1 of diuron, and 10 mg L-1 of atrazine. A synergistic effect between these two herbicides was noticed only for the lowest concentrations tested.