Abstract: The fry of mosquitofish (Gambusia affinis) were exposed to low concentrations of copper, mercury, cadmium, BHC and parathion in soft water. Tests were started with 2-day-old fry and lasted for 40 days. Total length and weight of mosquitofish were determined after 20 days and at the end of the tests. Under the test conditions described, the maximum allowable toxicant concentrations (MATC's) bare been obtained according to the effect on the growth of mosquitofish. MATC's estimated for copper, cadmium, and parathion were: 0.015—0.030, 0.005—0.010 and 0.0015—0.0030 mg/1 respectively. Their application factors were: 0.05—0.1, 0.0005—0.0009, and 0.01—0.05 respectively.