Experiments were conducted on various activated sludge samples over a span of several months to determine the feasibility of using cell DNA as an estimate of sludge biomass. These investigations demonstrated that free DNA released into a solution of viable microorganisms by lysis of cells was rapidly degraded. Therefore, DNA obtained from such cultures would effectively be DNA from viable organisms. Crude DNA was extracted from activated sludge by rupturing the cells by Sonication and centrifuging the sonicated sludge, heating at 70C to pasteurize the supernatant, and centrifuging again. Estimation of cell population was made by assuming the weight of one cell to be 1 x 10 to the -9th mg, and the DNA content of organic matter to be about 4% of the total weight of the organic matter. Population sizes based on DNA analyses were checked by using the MPN method. Values obtained were similar to values obtained with other methods.