The activation effects of diet-supplemented dimethyl-β-propiothetin (DMPT) on juvenile goldfish and rainbow trout were examined. Both fish were similarly reared on a commercial fish diet fed with small amounts of DMPT (117μg/g wet diet) and vitamin U (109μ/g wet diet) for only a short period of 9 days, then the resistance of the fish against oxygen deficiency (exposed to air for specified periods) and raised water temperature (0.54°c/min rising rate) was determined. Thus, the DMPT group fish of both fish exhibited the best growth and the strongest resistance against the oxygen-deficiency and the raised water temperature, when compared to those of the control and vtiamin U group fish. Accordingly, the dietary DMPT proved not only to stimulate the growth of fish but also to elevate their resistive power against environmental stress (oxygen deficiency and rising water temperature).