A numerical study of fatty acid patterns of 313 Bacillus strains was done based on standardized cultural and chemical techniques. All strains were characterized by the predominance of branched fatty acids (mainly il5:0, al5:0, often i17 :0, a17 :0, rarely i13 :0, i14 :0, i16 :0) ranging from 40% to more than 90% of total fatty acids. The ratios and proportions of the major fatty acids were found to be largely responsible for grouping strains into seven clusters (I to VII) by the UPGMA method using the correlation coefficient (S_C). Two main groups of Bacillus species can be defined on the basis of the ratio of the quantitatively predominant fatty acids i15:0 and a15:0. The species B. amyloliquefaciens, B. firmus, B. firmus-lentus intermediates, B. lentus, ‘B. freudenreichii’, B. laterosporus, B. licheniformis, B. megaterium, B. pasteurii, B. subtilis, B. alvei, B. circulans, B. coagulans, B. globisporus, B. lautus, B. macerans, B. pabuli, B. pantothenticus, B. polymyxa, B. psychrophilus and some strains of B. sphaericus (grouped into clusters I, II, and VII) are characterized by a ratio of i15 :0/a15 :0 < 2. The species B. thermoglucosidasius, B. badius, B. pumilus, the majority of B. sphaericus strains, B. stearothermophilus, B. cereus and B. thuringiensis grouped into clusters III to VI and exhibited a ratio of i15 :0/al5 :0 > 2. Significant quantitative variations were observed for particular fatty acids in different strains belonging to the species B. firmus, B. firmus-lentus intermediates, B. macquariensis, B. megaterium, B. subtilis, B. circulans, B. coagulans, B. macerans, and B. cereus. The species could be clustered into subgroups at a similarity level of 97.5% (S_c). With fatty acid analysis, Bacillus strains can be assigned to species groups, however, the enormous heterogeneity of fatty acid profiles within several species reflects the unsatisfactory taxonomic situation within the genus, which can be improved by further DNA-DNA hybridization studies.