Formalin is widely used for treating fungal infections of fish eggs in intensive aquaculture operations. The use of formalin in the United States is only allowed on salmonid and esocid eggs unless a special exemption is granted for use on other species. This study was conducted to determine the safety of formalin treatments on eggs of representative warm- and coolwater fish species and data was used to support a request to allow the use of formalin on the eggs of warmwater and additional coolwater fish species. Non-eyed eggs of walleye (Stizostedion vitreum), common carp (Cyprinus carpio), white sucker (Catostomus commersoni), channel catfish (Ictalurus punctatus), and lake sturgeon (Acipenser fulvescens) were cultured in miniature egg hatching jars and treated for 45 min every-other-day with 1500, 4500, or 7500 μL L-1 formalin up to hatch. For all species tested, the percent hatch was greater in 1500 mu L L-1 treatment groups than in untreated controls. Walleye eggs were the least sensitive species and had a hatch of 87% in the 7500 mu L L-1 treatment. Lake sturgeon were the most sensitive species with a mean hatch of 54% in 1500 mu L L-1 treatments. Adequate margins of safety exist for standard treatments (1500 mu L L-1 for 15 min) on eggs of all species tested except lake sturgeon. Fungal infections drastically reduced or eliminated hatch in most control groups whereas most treated groups were free of infections. This confirms the efficacy of formalin as an fungicide. Published by Elsevier Science B.V.