Five strains of Moraxella bovis were studied and found to yield distinct colony types with respect to agar corrosion and formation of corroding spreading zones. The types were largely similar to those of M. kingii, although some morphological differences were observed. Distinctly corroding clones, with more or less pronounced spreading colonies (SC type), were isolated from all strains. These isolates were all strongly fimbriated, as observed by electron microscopy. The fimbriae had a diameter of 65–85 Å. Another colony form, without corrosion and spreading (N type), was also observed in all strains. Electron microscopical preparations from these colonies revealed no fimbriae, or only occasional fimbriae, possibly of slightly different appearance, could be detected. With varying frequency, from none observed to about one in 10,000 colonies, N type populations revealed the intracolonial occurrence of fimbriated SC variants by partial change of colony morphology after a few days of incubation. Spontaneous variation from SC to N, accompanied by loss of fimbriation, was also observed. When 12–30 hr old SC colonies were subcultivated, less than one in 10,000 progeny colonies were usually observed to be of the N type, whereas older colonies sometimes revealed a high percentage of N variants in the progeny. A colony type intermediate between N and SC (NSC type) was occasionally seen, with relatively weak agar corrosion and spreading, and with an apparently slightly reduced degree of fimbriation, as compared with the SC type. The intermediate phenotype was unstable, however, showing an irregular tendency to produce progeny of a more corroding and spreading morphology. Two reference strains (including the neotype) of M. nonliquefaciens were studied and revealed fimbriae-associated colony type differentiation slightly different from previously examined strains of this species and from M. bovis.