[en] An ozone pilot plant was installed at the Dworshak National Fish Hatchery to examine the efficiency of sterilizing makeup water entering this recycle hatchery. The pilot plant consisted of two separate systems operated together. A recycle system consisting of two fish tanks, a clarifier, and bio-filter was in operation prior to this study. At the conclusion of the pilot plant study, an economic comparison was made of an ozone system and an ultraviolet system. Although the ozone treatment system requires a capital investment of $164,000 as opposed to $90,000 for an equivalent size ultraviolet system and an annual cost of almost $17,000 as opposed to $12,000 for the uv system, this study demonstrates that the increased cost may be justified. The ozone system gave consistently greater sterilization efficiency than the ultraviolet system