The large ciliate Paramecium cf. caudatum Ehrenberg was found to be a successful grazer of toxin producing Cylindrospermopsis in the laboratory. The feeding rate increased with increasing cell concentration to 1367 cell animal hr−1 at 4 1×105 cells mL−1 but declined slightly at cell oncentrations greater than this. Preliminary studies on the effects of this grazing on toxin concentrations in cultures of both straight and coiled forms of Cylindrospermopsis resulted in the production of different amounts of the toxin cylindrospermopsin in the different isolates. Differences in toxin production were also found between cultured and field populations from the lower Fitzroy River indicating that toxin production may be influenced by a suite of genetic and environmental factors. The proven ability of this ciliate to graze toxic Cylindrospermopsis provides some insight into interactions that may be able to control some toxic blooms in semiarid Australian conditions.