The RNA/DNA ratios in white muscle of carp were determined byUV spectroscopy as a physiological index of growth rate. The index was possibly usedto evaluate the effect of ecological environment and feeding condition on the carpgrowth. The experimental results indicated that the RNA/DNA values in whitemuscle of carp were correlated with the change in growth rate (r=0.8994)or theincrease in fish weight. Moreover, the RNA/DNA ratio was the highest in autumn,the lowest in winter and the medium in spring and summer time. When the ratio valuswas over 2.0, it showed that carp grew well, otherwise grew badly. The growth rate ofthe fish was significantly affected if the concentration of mercuric ion (Hg~(++)) reached0.005 mg/l, and this effect might be detected by measuring the RNA/DNA ratios inwhite muscle of the fish through UV absorbance measurements.