This study aimed to determine the influence of sewage sludge alkalization and acidification on the qualitative and quantitative composition of keratinolytic and keratinophilic fungi in a model experiment. The sludge was alkalized to pH 9 with 0.5N NaOH, 0.5N KOH, 0.5N NH₄OH or with 2% burnt lime mixed with powdered limestone (20g CaO + 5g CaCO₃), and acidified to pH 4 with 0.5N HCl, 0.5N HNO₃ or 0.5N H₂SO₄. The sludge with unmodified pH (6.5) served as control. The hair baiting method with four incubation temperatures (23, 29, 33, and 37°C) was used to examine fungi. The sludge alkalization with KOH, NaOH or CaO+CaCO₃ increased the number of keratinolytic fungi but decreased the number of keratinophilic fungi. The inhibition of the growth of keratinolytic fungi was observed due to the sludge acidification with H₂SO₄ or HCl. The sludge acidification with HNO₃ or HCl stimulated keratinophilic fungi to grow. Sludge acidification with HNO₃ eliminated keratinolytic fungi, while acidification with H₂SO₄ eliminated keratinophilic fungi. Results are discussed from ecological and epidemiological points of view.