A thermophilic and actinic bacterium strain, MH-1, which produced three different endochitinases in its culture fluid was isolated from chitin-containing compost. The microorganism did not grow in any of the usual media for actinomyces but only in colloidal chitin supplemented with yeast extract and (2,6-O-dimethyl)--cyclodextrin. Compost extract enhanced its growth. In spite of the formation of branched mycelia, other properties of the strain, such as the formation of endospores, the presence of meso-diaminopimelic acid in the cell wall, the percent G+C of DNA (55%), and the partial 16S ribosomal DNA sequence, indicated that strain MH-1 should belong to the genus Bacillus. Three isoforms of endochitinase (L, M, and S) were purified to homogeneity and characterized from Bacillus sp. strain MH-1. They had different molecular masses (71, 62, and 53 kDa), pIs (5.3, 4.8, and 4.7), and N-terminal amino acid sequences. Chitinases L, M, and S showed relatively high temperature optima (75, 65, and 75°C) and stabilities and showed pH optima in an acidic range (pH 6.5, 5.5, and 5.5, respectively). When reacted with acetylchitohexaose [(GlcNAc)₆], chitinases L and S produced (GlcNAc)₂ at the highest rate while chitinase M produced (GlcNAc)₃ at the highest rate. None of the three chitinases hydrolyzed (GlcNAc)₂. Chitinase L produced (GlcNAc)₂ and (GlcNAc)₃ in most abundance from 66 and 11% partially acetylated chitosan. The p-nitrophenol (pNP)-releasing activity of chitinase L was highest toward pNP-(GlcNAc)₂, and those of chitinases M and S were highest toward pNP-(GlcNAc)₃. All three enzymes were inert to pNP-GlcNAc. AgCl, HgCl₂, and (GlcNAc)₂ inhibited the activities of all three enzymes, while MnCl₂ and CaCl₂ slightly activated all of the enzymes.