The efficacy of sodium chloride was compared to that of formalin for improving egg hatch rate of koi, domestic mutation of the common carp Cyprinus carpio. Twelve-hour-old fertilized eggs were held in hatching trays and given a 60-min static treatment of 0, 1,000, 2,500, or 5,000 mg sodium chloride/L, or they were treated with formalin at 250 mg/L for 60 min or 1,000 mg/L for 15 min. Mean hatch rates for salt treatments at 1,000 and 2,500 mg/L and formalin treatments at 250 mg/L were 88.7%, 81.6%, and 98.8%, respectively; these rates were significantly higher than the hatch rate for untreated eggs. The mean hatch rate for untreated eggs was 64.0%, which was not significantly different from the rate for eggs treated with formalin at 1,000 mg/L for 15 min. Treatments with salt at 5,000 mg/L resulted in a mean hatch rate of 30.3%.