Abstract: Determination of fish abundance and chlorophyll a concentration were carried out in Labu River System and two selected location at Langat River. Collection of fish specimen has been done between November 1999 and November 2000. Examination of chlorophyll a and several selected physico-chemical parameters were conducted from December 1999 to July 2000. The effect of water quality to the existence selected aquatic organisms was the main approach of this study. Twelve hardy species of fishes that tolerance with polluted environmental have been recorded. The obtained result indicates that abundance of fish is independent with pH and temperature. However, Total Suspended Solid (TSS), ammonia-nitrogen (NH3-N) and dissolved oxygen (DO) have posed adverse impact on the distribution of fishes in the study ecosystem. Based on the Correlation Test performed shown linear correlation between chlorophyll a and pH was r=0.42, temperature (r=0.29), ammonium (NH4+) (r=0.52), nitrite (NO2-) (r=-0.16), nitrate (NO3-) (r=-0.10), phosphate reactive (PO43-) (0.39), phosphorus (P) (r=0.39), sulphide (S2-) (r=-0.07), sulpfate (SO42-) (r=0.63), calcium (Ca) (r=0.63), cadmium (Cd) (r=0.09), cuprum (Cu) (r=0.32), ferum (Fe) (r=0.30), magnesium (Mg) (r=0.47), manganese (Mn) (r=0.48), plumbum (Pb) (r=0.43) and zinc (Zn) (r=-0.02). TSS has not reveal significant impact to the detectable level of chlorophyll a in aquatic system