Effects of metal contamination on microbial biomass in sediment samples from three areas in Palestine Lake (one area highly polluted with chromium, cadmium and zinc) were determined . Adenosine triphosphate (ATP) concentrations, determined by the luciferin-luciferase bioluminescent technique, and microbial colony numbers on pour plates were used as biomass indicators .Plate counts showed a significant (P < 0 .01) site effect with the highly contaminated area having an order of magnitude lower microbial population than the control area . ATP concentrations also indicated lower microbial biomass in contaminated sediments . The metal concentrations of the most contaminated area averaged 17,840 µg Zn/g, 4380 µg Cr/g and 585 µg Cd/g based on dry weight of sediments. A suppression of organic decomposition was evident in the impacted area ; high metal levels and resultant low microbial biomass may have been causative .