The toxic diatom Pseudo-nitzschia seriata was exposed directly and indirectly (separated by a membrane)to copepods, Calanus hyperboreus and C. finmarchicus, to evaluate the effects of the copepods on domoicacid production and chain formation in P. seriata. The toxicity of P. seriata increased in the presence ofthe copepods. This response was chemically mediated without physical contact between the organismssuggesting that it was induced by potential waterborne cues from the copepods or changes in waterchemistry. Domoic acid production may be related to defense against grazing in P. seriata although it wasnot shown in the present study. To evaluate if the induction of domoic acid production was mediatedby the chemical cues from damaged P. seriata cells, live P. seriata cells were exposed to a P. seriata cellhomogenate, but no effect was observed. Chain formation in P. seriata was affected only when in directcontact with the copepods. This study suggests that the presence of zooplankton may be one of the factorsaffecting the toxicity of Pseudo-nitzschia blooms in the field.