A study of the cellular and colonial morphologies, the deoxyribonucleic acid (DNA) base composition, the degree of lysis by lysozyme, and some fundamental biochemical reactions was carried out on 23 strains previously identified as members of Bacillus megaterium. Included among these was NCTC 10342, the neotype strain of B. megaterium. The results showed that, primarily on the basis of their biochemical reactions, two of the strains belong to B. cereus, two belong to B. subtilis, and 19 belong to B. megaterium. The first two strains mentioned above clearly differed from B. megaterium strains with respect to their DNA base composition (33.2 and 34.3 mol% guanine plus cytosine [G+C]), their lysis by lysozyme (5.6 and 6.6%), and the morphology of their macro-and microcolonies. These properties were not helpful in differentiating the other two strains from the B. megaterium strains, for the two strains showed high G+C values (45.9 and 47.4 mol%) and high values of lysis by lysozyme, (92.9 and 96.7%). The 19 remaining strains, confirmed as members of B. megaterium, had G+C contents which varied from 37.3 to 43.0 mol% and lysis percentages which varied from 67.0 to 94.9. For these strains, a significant (P < 0.05) correlation was found between the DNA base composition and lysis by lysozyme. Three biogroups were established on the basis of the biochemical reactions of the 19 strains. These biogroups are similar to the "aggregate" and the "intermediate" strains of B. megaterium established by Gordon et al.In a previous paper, Candeli et al. (6) pointed out that three characteristics (colony shape, susceptibility to lysozyme, and deoxyribonucleic acid [DNA] base composition) were closely related to each other and were suitable for dividing seven of the Bacillus species studied into two groups. In the first group (B. subtilis, B. pumilus, B. licheniformis, and B. firmus), the rhizoid shape of the colonies was linked to a high degree of susceptibility to lysozyme and to a mean guanine-plus-cytosine (G+C) content of the DNA of 44.15 f 1.64 mol%. In the second group (B. cereus, B. cereus subsp. mycoides, B. anthracis, and B. thuringiensis), the "medusa head" shape of the colonies was related to a very low susceptibility to lysozyme and to a mean G+C value of 33.65 f 0.59 mol%. This taxonomic arrangement coincided (with few strain exceptions) with that presented for these species in Bergey's Manual (7) and based mainly on cell morphology and biochemical reactions. One exception involved the two strains studied which bore the name B. megaterium; one (ATCC 7481) was included in the fust group, and the other (NCTC 10342, the neotype strain of B. megaterium) belonged to neither of the two groups, although according to Bergey's Manual (7) it should belong to the B. cereus-B. anthracis-B. thuringiensis group. On the other hand, Bonde and Jackson (3) concluded that B. megaterium should be placed near B. subtilis, B. pumilus, and B. licheniformis (group C of these authors), particularly because of its DNA base composition.Our observa...