Aromatic chlorinated fungicides are potentially hazardous to environment and ecological equilibrium. This study investigates the biological breakdown properties of PCP (Pentachlorophenol) and PCNB (Pentachloronitrobenzene) aromatic chlorinated fungicides, using white rot fungi (Phanerochaete chrysosporium) to determine the degree of biological breakdown. The fungus was incubated at culture mediums of pH 5 and at the temperature 35 degrees C with basic nutrients and trace metals. The biological breakdown and rates of respective formation of corresponding materials were determined. At the end of 21 days incubation, degradation rate was found 40 to 100 percent for PCP, depending on concentration. On the other hand, at concentrations of 25 to 200 mu M, the PCNB was totally demolished at the end of 21 days incubation.