The effect of synthetic fungicides and fatty acids on the respiration of normal and pleomorphic forms of Trichophyton gypseum was examined manometrically. Organic thiomercuric fungicides, cationic detergents, and undecylenic acid markedly inhibited the respiration in as low as 10^-3 to 10^-5 molar concentration. Close relationship was observed between their respiration- and growth-inhibiting effects. The pleomorphic form was much more sensitive to the action of these compounds than the normal form of this fungus. Every saturated fatty acid of even-numbered series from C₂ to C₁₆ stimulated the respiratory rate of this fungus in a certain range of concentrations, while higher members among them above C₈ inhibited it in relatively higher concentration. The inhibitory action was stronger in the undissociated state than as anion of the same acid, and was the maximum with the C₁₂ acid. This dermatophyte was more sensitive to the action of these acids than bacterial species examined.