The toxic, estuarine dinoflagellate, Pfiesteria piscicida, has been implicated as a causative agent of major fish kills along the eastern coast of the U.S.A. Pfiesteria zoospores are unique in that they exhibit directed attack behavior towards live finfish and produce toxin(s) which strip epidermal tissue from finfish and impair the nervous system, leading to paralysis and suffocation. Here we present the first report of directed attack behavior by P. piscicida zoospores toward larval pediveligers of the bay scallop, Argopecten irradians. A weaker attack response toward pediveligers of the eastern oyster, Crassostrea virginica, was also observed. Within 5 minutes of zoospore introduction into culture with larval pediveligers ( 21 oC, 15 psu, zoospores were observed to congregate around the individual larvae and attach via their peduncles. Within 15 minutes, the zoospores had penetrated into the visceral cavity of the shellfish larvae and had begun to feed aggressively upon exposed tissue. After 30 minutes, all shellfish tissue except the adductor muscle had been consumed, leaving a hollow cavity in which the zoospores encysted. Only the pediveligers which had discarded their velums were preyed upon by the zoospores. Those larvae with active, extended velums appeared to discourage the zoospores‚ attack and feeding behavior. In contrast to these observations with larvae, adult shellfish were observed to actively filter Pfiesteria zoospores out of suspension. Examination of the faeces produced by the shellfish indicated that the zoospores had formed temporary cysts and passed through the digestive tract with no apparent adverse affects on cell viability. Within three hours, 90% of the cysts excysted and regained motility. The data indicate that Pfiesteria zoospores have the potential to adversely affect larval recruitment and survival.