Differential identification of Trichophyton rubrum and Trichophyton mentagrophytes was evaluated by using the standard in vitro hair perforation test as a criterion for identification and comparing its effectiveness with that of nine other tests used to differentiate these two taxa. Correlation and multiple regression analysis showed that no single test or combination of these tests was able to make the differentiation as effectively as the standard in vitro hair perforation test. Using relative error analysis, we suggest an identification scheme based upon selective utilization of potato-carrot agar for sporulation and the standard in vitro hair perforation test. Using potato-carrot agar for development of typical microconidia on each of the nonsporulating primary isolates and subsequently performing the standard in vitro hair perforation test only on those isolates that failed to produce spores on potato-carrot medium, we showed that this combination scheme provides more expedient results, without any sacrifice in reliability, than