ABSTRACT: A Vibrio anguillarum-related (VAR) strain, isolated in pure culture from an epizootic in a commercial hatchery producing Argopecten purpuratus, was characterized, and its potential pathogenicity to veliger larvae of A. purpuratus determined. Experimental challenges indicated that the bacterium affects larval survival at concentrations of 10⁴ to 10⁸ cells ml^-1. The effect of water quality and temperature on pathogenicity was also evaluated. Larval survival in seawater filtered through 5 um pore-size membranes was 45.6%, whereas using seawater passed through 1 and 0.2 um filters, larval survival increased to 66.4 and 80.4% respectively. Temperature also affected pathogenicity as larval survival at 15*C for 24 h was 69.3% but decreased to 30 and 26.9% at 20 and 25*C, respectively. Toxic activity was found in cell-free supernatant of bacterial culture. Larval survival was reduced to 68.9 and 36.4% after 20 and 40% (v/v) of supernatant, respectively, was added to the rearing water. These results suggest that exotoxins produced by the VAR strain play an important role in its pathogenicity for scallop larvae.