In four out of five Moraxella kingii strains examined, strongly corroding and usually spreading colony forms (named SC type) could be isolated, as well as non-spreading and non-corroding or weakly corroding forms (N colony type). Electron microscopical preparations made from SC colonies always contained numerous fimbriae, whereas usually no fimbriae could be detected in preparations made from cultures of the N type. In the case of weakly corroding forms, a few fimbriae could be observed in some preparations, their occurrence apparently corresponding in time with the slowly developing corrosion. The fimbriae of M. kingii were of the same approximate dimensions (60–85 Å) as observed for spreading-corroding colony variants of M. nonliquefaciens and M. bovis. In one strain, low-frequent spontaneous variation from the N to SC type was observed, with a corresponding gain of fimbriation. Spontaneous variation in the other direction, from SC to N, accompanied by loss of fimbriation, was also seen in this strain. The latter variation was probably also always of a low frequency. When it had occurred, the N variant cells easily outnumbered the SC type in mixed culture.