The incidence of keratinolytic fungi in an acidic petroleum waste lagoon (before bioremediation) at a petroleum refinery situated within a highly populated area was examined. High concentrations of petroleum hydrocarbons (aliphatics and PAHs) made the growth of keratinolytic fungi in clay and litter collected from the lagoon impossible. The natural self-purification process considerably decreased the hydrocarbon contamination, increased the pH and caused the abundant growth of Trichophyton ajelloi in organic soil that contained the root-adjacent material from the grass growing in green oases at the lagoon. Ecological and epidemiological aspects of the data were discussed.