The Asian oyster, Crassostrea ariakensis, is being evaluated for its potential success in the restoration of oyster populations in Chesapeake Bay. Critical to an understanding of its potential success is knowledge of the impacts of common harmful algae in its diet; blooms of such algae are common in Chesapeake Bay. In these experiments, C. ariakensis were exposed to a standard algal diet, Isochrysis sp., alone, and in combination with the harmful dinoflagallates Prorocentrum minimum and Karlodinium veneficum and the raphidophytes Heterosigma akashiwo and Chattonella subsalsa. Two experiments were run, with varying proportions of Isochrysis to the test algal species. Feces and pseudofeces were examined microscopically and by high-performance liquid chromatography for changes in diagnostic pigments relative to the initial diet and for production of degradation pigments of chlorophyll. All species were cleared from suspension to varying degrees by the oysters. In the Isochrysis control and in the Isochrysis P. minimum treatment, intact, solid feces were produced, and the highest proportion of chlorophyll degradation pigments were found in the Isochrysis control diet suggesting algal digestion. Thin, “ropey” feces and pseudo-feces were observed in the K. veneficum Isochrysis treatment. Virtually no degradation pigments were found for oysters fed a diet containing K. veneficum, suggesting lack of digestion and assimilation of algal food. With the raphidophytes, reduced production of feces and pseudofeces was evident, and these contained a lower proportion of recognizable harmful algal cells, and higher proportion of degradation pigments than found with the other test species. Amorphous and cellular material that appears to be partly derived from the oyster digestive system was evident in the feces of oysters exposed to K. veneficum, H. akashiwo, and C. subsalsa; this was particularly pronounced in oysters exposed to H. akashiwo and suggests damage to the oyster gut. The impact of the presence of harmful algae in the diet of the oysters thus varied by algal species, but in all cases oyster digestion was altered relative to the control diet.