Oxolinic acid {5-ethyl-5, 8-dihydro-8-oxo-1, 3-dioxolo [4, 5-g]-quinoline-7-carboxylic acid} (OA) was mixed in the feed and administered to fish for 5 days at the rate of 20mg of OA per kilogram of body weight per day. The tissue levels of OA in cultured rainbow trout (Salmo gairdnerii) and ayu fish (Plecoglossus altivelis) were determined by high performance liquid chromatography and the following results were obtained. 1. OA residue was detectable in tissue of rainbow trout for longer in light body weight (average 10g) fish than in heavier fish (average 160g). 2. OA residue was detectable in tissue of rainbow trout for longer at high water temperature (17.0-19.6°C) than at low water temperature (8.5-11.5°C). 3. OA residue remained for a longer period in the tissue of rainbow trout than in that of ayu fish. 4. The existing withholding period of OA administration prior to consumption of rainbow trout and ayu fish is not sufficient; about 21 days for rainbow trout and about 14 days for ayu fish are necessary.