The release of oxygen from hydrogen peroxide, with and without catalysts, was assessed using the Winkler technique and an oxygen electrode. It was shown that the Winkler technique over-estimates dissolved oxygen when used with hydrogen peroxide. Both fish liver and blood were shown to be very active field catalysts of hydrogen peroxide. The toxicity of hydrogen peroxide to fry of Oreochromis niloticus was measured and a dose level of 0.15 ml/l of 12% w/v H 2 O 2 was found to produce no clinical toxicity. A simple and cost-effective system for transporting fry using oxygen derived from hydrogen peroxide is described in which the oxygen is generated separately from the fish container.