In April 1993, a phytoplankton dinoflagellate was isolated from Hiroshima Bay, Hiroshima Prefecture, Japan, and unambiguously identified as Alexandrium tamarense on the basis of the morphological characteristics. The dinoflagellates, cultures in modified SW-2 medium at 15 degrees C for 15 days, showed a specific toxicity of 30.7 x 10-6 MU/cell. HPLC analysis demonstrated that the toxin was composed mainly of gonyautoxin-4 (GTX4) and protogonyautoxin-2 (PX2 or GTX8)(27.6 and 37.0 mole%, respectively). Total toxin concentration of this strain was 39.5 fmole/cell. Short-necked clams, mussels, and oysters contaminated by the dinoflagellate showed a more complicated composition, with GTX1 as the major component (61.8 mole% for short-necked clams, 60.5 mole% for mussels, 42.5 mole% for oysters), and PX2 was only present in trace amounts.