In order to detect phosphatase activity a total of 137 isolates from 12 Clostridium (C.) species were examined via fluorescence on SCA-agar with methylumbelliferyl phosphate (SCA-MUP), via APIZYM and RAPID ID 32 A as well as using a phosphatase reagent containing 1-naphthyl phosphate. Fluorescence on SCA-MUP showed the presence of acid or alkaline phosphatase in almost all isolates examined. Likewise, acid or alkaline phosphatase could be detected via APIZYM and RAPID ID 32 A in most strains and species. Opposed to this, the majority of the Clostridium bifermentans isolates showed a positive reaction exclusively on SCA-MUP. On the other hand, the phosphatase reagent for the detection of acid phosphatase lead to unambiguously positive results only when examining Clostridium perfringens isolates. Therefore, the SCA-MUP-agar, in contrast to the phosphatase reagent, was proven to be unsuitable for the identification of C. perfringens via detection of acid phosphatase. Using the phosphatase reagent the activity of this enzyme was detected in 95.1% of the C. perfringens isolates included in the study. In addition, the phosphatase reagent showed identical reactions after a 24 h incubation at 37 degrees C and when used on cultures incubated for 6 h at 44 degrees C in the case of 98.5% of the C. perfringens isolates.