Marine fishes (Atlantic herring, American pollock, winter flounder, Atlantic salmon, and cod) were dosed orally and intraperitoneally (i.p.) with “paralytic shellfish toxins” extracted from Bay of Fundy Gonyaulax excavata (tamarensis) cells. The toxins are lethal to these fishes in low oral doses, and in extremely low i.p. doses. Symptoms are the same among these fishes, both for oral and i.p. administrations, including loss of equilibrium within 5 to 15 min, followed by immobilization and shallow, arrhythmic breathing. Death generally occurs within 20 to 60 min of toxin administration. Dose responses are also similar among these fishes. Oral LD50 values are 400 to 750 μg saxitoxin (STX) equivalent kg-1 body weight. Intraperitoneal LD50 values are 4 to 12 μg STX equivalent kg-1. Toxins are undetectable in fish muscle tissue following lethal oral doses. The similarity of symptoms and dose responses suggest that fish as a group are sensitive to G. excavata toxins. Results, in combination with reports implicating these toxins in herring, sand lance, and menhaden kills, show the plausibility that the nearly worldwide blooms and red tides of G. excavata and its relatives may cause kills of a variety of fishes.