Feeding, respiration, and ammonia excretion of Acartia tonsa were studied in a brackish lagoon, near Marseilles (Southern France). The experiments were performed during the same season in two successive years, using naturally occurring particles as food. Strong variations in both quantity and quality of food particles were recorded between the two series of experiments. The ingestion of particles according to their size was studied by means of the Coulter Counter technique. Females ingested more food than males. In both sexes, there was a tendency to select median to large particles, regardless of the shape of the particles-size spectrum. In 1990, the specific production rate, deduced from the energy budget, was equivalent to or greater than the observed egg production, while in 1989 it was too small to account for the observed rate. This could be due to the poor nutritive value of particles in 1989, as evidenced by their low chlorophyll:volume ratio. The nutritive requirements would then have been completed by ingestion of nauplii and rotifers which were particularly abundant in 1989. This presumed shift to carnivorism is borne out by the lower O:N atomic ratio (oxygen consumed and NH4-N excreted) calculated for this period.