Summer outbreaks of Gyrodinium aureolum in European coastal waters have been described since 1966 (Braarud & Heimdal, 1970). The coincidence of G. aureolum blooms and observations of noxious effects on fauna (Widdows et al., 1979; Tangen, 1977; Jones et al., 1982; Southgate et al., 1984; Potts & Edwards, 1987; Thain & Watts, 1987) is well documented. However, great differences in toxic effects have been found in in vitro studies (Partensky & Sournia, 1986), with critical cell densities varying from 6×105 cells 1−1 to 3×107 cells 1−1. Production of a lethal agent has been suggested by Turner et al. (1987). More recently, the toxic principle has been identified by Yasumoto et al. (1989).