Physiological and chemotaxonomical studies were carried out on microorganisms isolated from a composter operated at around 50 to 60 degrees C. Only gram-positive rods were isolated and the microbial counts on agar plates were relatively low (10(3) to 10(5) cfu/g-sample). The chemotaxonomic properties of the ten isolates revealed that they were typical of the genus Bacillus. Identification by the Biolog system concurred with the chemotaxonomy; all the strains were determined to be Bacillus. 16S rDNA analysis closely identified five of the isolates as B. licheniformis, one as B. subtilis, and two as B. thermoamylovorans, while the remaining two strains seemed to belong to a new species. However, in several cases 16S rDNA analysis and the Biolog system gave different results at the species level. The stability of microflora within the composter was also investigated using the Biolog system. Analysis of colonies obtained during three sampling periods-December 1997, April 1998, and June 1998-showed that the microflora in the composter remained fairly stable.