Seventeen day old rainbow trout fry,Oncorbynchus mykiss, were subjected to four feeding regimes during 13 days at‐10° C: continuously fasted; delayed fed after 6 days fasting; fedad libitumwith Trout Chow #3 three (at 08: 00, 12: 30 and 17: 00 h); or six (08: 00, 10: 00, 12: 00, 14: 00, 16: 00, and 18: 00 h) times a day. Total RNA and DNA concentrations of the fry were measured by fluorometry at two day intervals. Fed fry continuously grew from ∼ 99 mg to over 200 mg after 13 days, but starved fish underwent growth loss of‐73 mg. Their condition factors (K) increased from ∼2.2 to ∼7–8 for the fed, and decreased to ∼1.9 for the fasted fish. Specific growth rate per day (SGR) increased from 0.05 mg to‐33.6 mg for the continuously fed fry six times a day (average 9.8% per day) and 29.7 mg for the fry fed three times a day (8.5%), but‐1.15 mg for the fasted fry (‐1.96%). Fed fish starved for 6 days, recovered their SGR after three days following feeding (day‐9), but did not catch up to the continuously fed fish (‐0.73%). DNA concentrations increased from‐450 μgmg per fish to‐600 μgmg for the starved fish,‐440 μg mg for the delayed fed, and‐290 μg mg for the continuously fed fish. RNA concentrations were ∼466 μg‐mg for the starved,‐840 μgmg for the re‐fed, and‐960 μg. mg for the fed fry. RNA/DNA ratios changed from 1.72 to 0.73 for the starved, 1.96 to 1.91 for the delayed feeding, and 1.8 to 3–3 for the fed three times a day, and 1.8 to 3.4 for the fed six times a day after 13 days. No significant differences existed between the fish fed three and six times a day. The results confirmed that SGR, total weight increment, and K were all higher in fed fish than in starved, indicating that feeding has a great influence on RNA/DNA ratio as physiological index in rainbow trout fry.