The present study evaluated the possible effects of a toxic dinoflagellate, Karenia selliformis, upon immunological hemocyte functions of the Manila clam Ruditapes philippinarum, and on the progression of infection by Perkinsus olseni. Clams with variable levels of perkinsosis were exposed for 6 weeks to simulated blooms of cultured the K. selliformis (102 and 103 cell ml-1). Samples were collected after 0, 2, 3, and 6 weeks of exposure. The following hemocyte parameters were measured by flow cytometry: percentage of dead cells, cell size and complexity, apoptosis, phagocytosis, and production of reactive oxygen species. Agglutination activities of K. selliformis on horse erythrocytes, serum protein concentration, and condition index of clams were also assessed. The harmful alga K. selliformis caused a significant decrease in hemocyte size and percentage of apoptotic cells. In contrast, P. olseni did not affect clams strongly; the only significant effect was an increase in hemocyte size in heavily infected clams. After 2 and 3 weeks, the prevalence and burden of P. olseni decreased in clams exposed to K. selliformis, but after 6 weeks, and a diminution in K. selliformis cell density in the exposure, this effect disappeared. In vitro tests exposing P. olseni to K. selliformis showed direct algal toxicity to the parasite (increased percentage of dead cells and altered morphology). Initial exposure of P. olseni-infected clams to K. selliformis appeared to modify the host-parasite interaction by causing effects in both organisms.