A combined genetic transformation and nutritional assay is described that permits definitive identification of clinically isolated strains of Moraxella nonliquefaciens. Crude DNA preparations of strains of various Moraxella species were used to transform nutritional mutants of a stably competent strain of M. nonliquefaciens for ability to grow on a defined medium (Mn-B). DNA samples from 24 independently isolated strains of M. nonliquefaciens all resulted in massive (4+) transformation of each of two mutant assay strains. DNA samples from strains of M. bovis and M. lacunata frequently gave weak (1+) transformation of one of the mutant assay strains (Mn64) but almost always failed to transform another assay strain (Mn136). DNA samples from eight other Moraxella species failed completely to transform either of the mutant assay strains. When streaked on the defined medium used for the transformation assay (Mn-B), 23 of the 24 strains of M. nonliquefaciens grew well, but all strains of M. bovis and M. lacunata failed to grow on this medium.