The antifungal effects of potassium chloride (KCl) against Saprolenia diclina, S. parasitica and Achlya sp. and fungal infection in ayu Plecogrossus altivelis eggs were evaluated. Zoospore motility of the fungi was greatly inhibited within 1 min in 0.03% KCl, although hyphal growth and zoospore germination of the fungi were not inhibited at a concentration of 0.24% KCl. When fertilized ayu eggs were continuously kept in 0.03%, 0.06% and 0.12% KCl until the eyed-egg stage, treatment at concentrations of 0.06% and 0.12% apparently decreased the rate of fungal infection in three separate experiments. These concentrations showed no effect on the egg-eyed ratio, the subsequent hatching and the larval deformity rates. It was confirmed that KCl was a useful chemical to prevent mold infection in ayu eggs.