Four thousand rainbow trout eggs (in 2 age groups) were hatched under poor conditions of hygiene in 20 gal. of completely re-circulating water. All dead eggs, dead fry and attendant fungal growth were retained in the system. Eighty-four per cent of the 1st group was reared to advanced fry, 34% of the 2nd was hatched to sac fry. The remarkably high yields are attributed to the use of ozonized air to satisfy 02 demand and control bacteria and Saprolegnia. A re-examination of the putative toxicity of ozone to fish seems indicated.