In 1993 and 1994 a highly increased occurrence of equine dermatophilosis was observed, and a study was initiated to determine phenotypic heterogeneity among 120 clinical isolates using biochemistry, antibiotic resistance profiles, membrane protein profiles and Western blotting. The biochemical examinations contained 1% equine serum in medium. Moreover, the API ZYM-test from bioMérieux was used. The biochemical reactions were suited to identify Dermatophilus congolensis but did not allow a differentiation among the various isolates. Antibiotic resistance in one or more isolates was observed against polymyxin B, enrofloxacin, oxacillin, neomycin and trimethoprim-sulfamethoxazol. All isolates were sensitive penicillin G, ampicillin, streptomycin, gentamicin, lincomycin, erythromycin, tetracycline, oxytetracycline, bacitracin and ceftiofur. The evaluation of silver-stained and immuno-stained membrane protein profiles showed minor differences among several isolates. In total, all isolates appeared to be closely related and the minor differences observed did not correlate with the geographic origin of the respective isolates.