ABSTRACT: Aquaculture and restoration activities with bivalve molluscs often involve moving individuals from one body of water to another. Our study tests the hypothesis that harmful algae ingested by source populations of shellfish can be introduced into new environments by means of these shellfish relocations. Cultures of several harmful algal strains, including Prorocentrum minimum, Alexandrium fundyense, Heterosigma akashiwo, Aureococcus anophagefferens, Karenia mikimotoi and Alexandrium monilatum, were fed to various species of bivalve molluscs, Crassostrea virginica, Argopecten irradians irradians, Mercenaria mercenaria, Mytilus edulis, Mya arenaria, Venerupis philippinarum and Perna viridis, to assess the ability of the algal cells to pass intact though the digestive tracts of the shellfish and subsequently multiply in number. Ten individuals of each shellfish species were exposed for 2 d to a simulated harmful algal bloom at a natural bloom concentration. The shellfish were removed after exposure, and maintained for 2 further days in ultra-filtered seawater. Biodeposits (feces) were collected after 24 and 48 additional hours, and observed under light microscopy for the presence or absence of intact, potentially viable algal cells or temporary cysts. Subsamples of biodeposits were transferred into both algal culture medium and filtered seawater and monitored for algal growth. Intact cells of most harmful algal species tested were seen in biodeposits. Generally, harmful algae from the biodeposits collected in the first 24 h after transfer re-established growing populations, but algae could less often be recovered from the biodeposits collected after 48 h. These data provide evidence that transplanted bivalve molluscs may be vectors for the transport of harmful algae and that a short holding period in water without algae may mitigate this risk. Further, preliminary results indicate that emersion may also serve to mitigate the risk of transport.