A study was undertaken in which both freshwater and marine fish were chosen as indicator organisms to monitor pollution by heavy metals (copper and zinc). Experiments were conducted to determine the adaptability of guppies to various salt concentrations. Juvenile and adults guppies were subjected to both abrupt and gradual changes from freshwater to various salinities. Both could tolerate gradual transfer from freshwater (S=< 1%, ) to 30%. salinity, but mortality did occur when they were transferred directly to different salinities. Juvenile guppies are more sensitive to copper and zinc than the adults. The survival rate was found to be increased with increase in salinity; the LC50 for both adalt and juvenile guppies increases with increasing salinity. The same results were found in a marine goby, Pomatoschistus minutus. The effect of heavy metals were also examined on the rate of oxygen consumption in both guppies and sticklebacks. Adding copper to the zinc solution used lowars the rate of oxygen consumption in sticklebacks but gives a range of values for the rate of oxygen uptake in guppies. Generally, increasing the zinc concentration decreases the oxygen consumption in both species. Plaice and dab were found to accumulate copper and zinc unevenly in different organs. Liver accumulated the highest concentration of accumulated zinc for both dab and plaice. In the eight species of fish examined, the liver displayed the highest concentration of zinc by comparison with the gills and muscles.