The Eastern oyster Crassostrea virginica and the Asian oyster C. ariakensis are native and introduced oysters, respectively, in the Chesapeake Bay and the northern quahog (hard clam) Mercenaria mercenaria, common to sandy habitats of the Bay and adjacent coastal Bays, is a major cultured species in Virginia waters. Each species will be exposed to the phytoplankton assemblage which includes harmful species throughout their life cycles. Recent work suggests that at least one prey item, the ichthyotoxic dinoflagellate Karlodinium veneficum, occurs frequently throughout the range for the oysters and clam and produces toxins (karlotoxins, KmTX) which elicit toxicity through formation of sterol-dependent, non-specific membrane pores. Juvenile oysters and clams were exposed to moderately toxic strains [18.5 ± 6.2 ng KmTX mL-1] at environmentally-relevant cell densities. Juvenile oyster growth rates were significantly reduced when feeding on Karlodinium as were clearance rates for both oysters and clams relative to a nontoxic prey species. These initial results suggest that the cosmopolitan Karlodinium veneficum could substantially alter oyster recruitment as well as temporarily curtail seston clearance in blooms areas.