Growth and ingestion by the loricate ciliate Amphorides quadrilineata exposed to increasing dietary doses of the Texas brown tide alga Aureoumbra lagunensis were investigated. The ciliate grew at a maximum rate of 0.38 d(-1), ingesting 0.032 ppm (similar to6.4 x 10(2) cells) prey d(-1) on a diet consisting only of Isochrysis galbana. When A. quadrilineata was offered a mixed diet of I. galbana and A. lagunensis, the growth rate decreased when A. lagunensis made up more than ca 40 % of the diet. Ingestion rate by A. quadrilineata remained constant with increasing A. lagunensis proportion in the diet, which indicates that A, quadrilineata did not avoid feeding on A. lagunensis. The behaviour of A. quadrilineata was largely unchanged when ciliates were fed A. lagunensis. However A. quadrilineata did perform a significant transient response approx. 60 min after A. quadrilineata was exposed to A. lagunensis by increasing the turning rate per unit time. This study suggests that efficient top-down control of A. lagunensis by heterotrophic protozoans such as the studied ciliate may not happen as long as phytoplankton organisms other than A. lagunensis make up a minor part of the standing phytoplankton stock.