ABSTRACT: The fate of paralytic shellfish poisoning (PSP) toxins ingested by the copepod Acartia clausi was studied in unialgal and mixed cultures of the toxic dinoflagellate Alexandrium minutum and the non-toxic dinoflagellateProrocentrum micans. Acartia clausi fed actively on Alexandrium minutum, but feeding pressure diminished over time. This reduced feeding upon toxic phytoplankton seems to be due to behavioural rejection, since feeding pressure on thenon-toxic dinoflagellate did not diminish over time. The assimilation efficiency of toxins ingested by copepods was 3.8%. Some of these toxins assimilated by copepods were redirected to the eggs, but the daily total toxin output in the eggs was only 0.98%of the daily toxins assimilated by the copepods. This small amount of toxins in the eggs had no effect on the fate of the toxins in the copepods, but did affect copepod reproductive success, since reduced egg hatching was observed with increasing toxinaccumulation in the copepod tissues. The amount of toxins daily excreted in the pellets was only 2.26% of the daily amount of toxins assimilated by the copepods. However, the detoxification rate of PSP toxins by the copepods was 0.586 d^-1.Therefore, toxins were either transformed and excreted as other compounds in faecal pellets and/or were eliminated through excretion in dissolved form. A model showed that the copepods accumulated PSP toxins through dietary incorporation, but excretedthem after several days. Copepods accumulate toxins up to a threshold without any negative effect on fecundity, but above this threshold, they require a higher amount of food to achieve the same egg production rate.