Urease activity was detected in the dermatophyte Trichophyton mentagrophytes cells at early exponential phase of growth. Specific activity of urease decreased with culture age. At exogenous urea concentrations above 2 mm formation of urease was inhibited. The pH optimum lay at 7-7.5, the Km being 14 mm. No urease activity could be detected in cell-free culture fluid of T. mentagrophytes. No endoor exocellular urease activity could be detected in a T. rubrum strain grown with or without urea. © 1983 Institute of Microbiology, Academy of Sciences of the Czech Republic.