Acute toxicity of metals viz., iron, zinc, lead, nickel and manganese to the fish, Catla catla has been studied. These tests included the determination of 96 h LC50 and lethal toxicity of heavy metals to the fish. These tests were performed, separately, at constant temperature, pH and hardness of 30°C, and 100 mg L-1 respectively. Three fish age groups viz. 30, 0 and 90 day were tested for their sensitivity to metals toxicity. The impacts of physico-chemical variables were also studied towards sensitivity of fish to metals toxicity. The 96 h LC50 and lethal concentrations of all metals varied significantly in fish. This fish showed significantly highest tolerance (determined as LC50) against iron, followed by that of manganese, lead, zinc and nickel. However, non-significant differences for 96 h LC50 tolerance limits towards zinc and lead were found. Among the three fish age groups, 0 day fish were more sensitive to metals toxicity, followed by that of 60 and 90 day respectively. The responses of three fish age groups and five metals were statistically significant. Among the three age groups, 0 day fish showed significantly higher tolerance against all metals than that of 60 and 30 day fish. The ammonia excretion by the fish increased, significantly, with concomitant increase in metal concentrations of the mediums while dissolved oxygen content of the test medium decreased at higher metal concentrations. Sodium had slight protective effect against metals toxicity.