Salmonid eggs are highly sensitive to physical shock during the period from fertilization to eyed stages. In hatcheries, once eggs are set into incubating trays, they are not moved even in cases where some of them die during those stages. Therefore, if there is no effective prophylactic and/or therapeutic treatments applied to the incubating eggs, an aquatic fungus infects the dead eggs, spreads rapidly, forms fungal clumps engulfing the adjoining live eggs, and then often causes high egg mortality due to oxygen deficiency. Malachite green is quite an effective antifungal agent, but in Japan the Pharmaceutical Affairs Law has prohibited the use of it since 2003. An effective alternative antifungal agent is needed. The antifungal activities of sodium hypochlorite (NaOCl), a food additive, have been investigated, and these have suggested that NaOCl would be a promising agent to replace malachite green. However, further research is required to make it fit for practical use because it may bring about the undesirable effects resulting in egg softening. The softening seems to be a major reason why NaOCl has not been discussed as a suitable drug for fishery use, though the actual harm of softening is not clear. Under these circumstances, Kashiwagi et al. indicated that eggs of rainbow trout Oncorhynchus mykiss did not soften with NaOCl treatment at 10 mg/L with daily 15-min exposure. It has been known that the same treatment with NaOCl is effective in controlling fungal infections on the eggs of chum salmon Oncorhynchus keta, and maintaining a higher percentage of their eying. The purpose of this study is to determine the extent of chum salmon egg softening caused by NaOCl treatment at the same dose. The experiments were carried out at the Tsukidate Salmon Hatchery, Miyagi Prefecture, Japan. Unfertilized eggs were stripped from four adult females caught in Hasama river. The eggs from each female were separately fertilized in situ with sperm squeezed from several adult males, and transported to the hatchery. The eggs of each female were divided into two trays each containing approximately 500 eggs, and then separately set into the two tanks for NaOCl treatment and the untreated control, receiving water with a flow rate of 20 L/min. Water temperatures and dissolved oxygen ranged from 8.0 to 14.5°C and 12.2– 16.0 mg/L, respectively, during the incubation. Commercial grade NaOCl (Wako Pure Chemical, Osaka, Japan) was used for the treatment that started from the next day of fertilization (incubation). NaOCl of 2.6 mL/min was administered into incubation water by a pump (Masterflex Micorprocessor Peristaltic Pump 7524-40; Cole-Parmar, Vernon Hills, IL, USA). Target chlorine concentration of 10 mg/L was achieved 10 min following the initiation of treatment and maintained during a 15-min period. The residual chlorine concentration was checked with a DPD (N, N-diethyl-pphenylenediamine) test kit (Pocket Colorimeter 4670-00; Hack, Loveland, CO, USA). On day 23 after fertilization, as the eggs developed into the eyed stage at which time the eyes became clearly visible, the treatments were terminated. Thirty eggs at the eyed stage were randomly picked from each tray and the hardness of them measured. The egg diameter ranged from 6.3 to 7.9 mm in diameter (mean 7.2 mm). To measure the hardness, the *Corresponding author: Tel: 81-59-231-9526. Fax: 81-59-231-9540. Email: motoi@bio.mie-u.ac.jp Received 18 October 2005. Accepted 10 April 2006. FISHERIES SCIENCE 2007; 73: 1205–1207