This study was conducted to investigate the antimycotic activities of eugenol, a major essential oil of clove, against some fish pathogenic species of Saprolegniaceae as well as to determine the toxicity of eugenol to selected cultured fish. Two eugenol solutions were used in this study, a 10% v/v (volume:volume) solution in dimethyl sulfoxide (DMSO) and FA 100, which contains 10% v/v eugenol. The fungal species investigated were Saprolegnia parasitica, S. diclina, S. ferax, S, salmonis, Achlya klebsiana, and Aphanomyces piscicida. The minimum inhibitory concentrations (MICs) of eugenol in DMSO against Saprolegnia spp., ii. klebsiana, and A. piscicida were 500, 250, and 125 mug/mL, respectively, and the fungicidal concentrations (for complete kilting) were 1,000, 500, and 250 mug/mL, respectively. In contrast, the MICs of FA 100 against S. parasitica, other Saprolegnia spp., A. klebsiana and A. piscicida were 250, 125, 250, and 63 (mug/mL, respectively, and the fungicidal concentrations were I,1,000, 1,000, 1,000, and 63 mug/mL, respectively. Zoospores of the Saprolegnia spp. tested and of A. klebsiana could not germinate in the presence of a concentration of 250 mug/mL of either eugenol solution, while those of A. piscicida were killed by a concentration of 125 mug/mL of either eugenol solution. Eugenol was highly toxic to salmonids but less toxic to cyprinids.