The effects of the exposure of two tintinnids (Favella taraikaensis and Eutintinnus sp.) to the toxic dinoflagellate Alexandrium tamarense were studied, using the non-toxic dinoflagellate Scrippsiella trochoidea as a control. The tintinnid F. taraikaensis showed backward swimming and cellular lysis after a few minutes in the presence of the toxic dinoflagellate A. tamarense in high concentration (280–700 cells ml-1). At low concentrations of A. tamarense (30–120 cells ml-1) no swimming alterations were observed, but cellular lysis occurred after 48 hours. No changes in the swimming patterns were observed when Eutintinnus sp. was exposed to either toxic dinoflagellates or cell-free filtrate, or when F. taraikaensis was exposed to cell-free filtrate. However, both tintinnids suffered cellular lysis after 48 hours of exposure. Favella taraikaensis and Eutintinnus sp. remained healthy after preying on the non-toxic S. trochoidea for 48 hours. In selectivity experiments, F. taraikaensis avoided cells of the toxic dinoflagellates during the dark phase. Our results suggest that tintinnids could play a regulatory role when the toxic blooms are initiating and the toxic cell concentrations remain still low in the northern Patagonian gulfs of Argentina. Since tintinnids are affected by substances released by A. tamarense, predation on dinoflagellates would not occur during the peak of the bloom.