Seven isolates of an unclassified bacterium resembling Flavobacterium spp. were characterized by growth requirements, microscopic examination, biochemical characteristics, antimicrobial susceptibility tests, protein profile analysis, and serologic data. The unclassified isolates were differentiated from Flavobacterium meningosepticum, Flavobacterium odoratum, Flavobacterium balustinum, Flavobacterium strain IIb, Chromobacterium violaceum, Aquaspirillum serpens, and Pseudomonas spp. The bacterium was a gram-negative rod with a polar flagellum. Protein profile analysis demonstrated two major protein bands present in the unclassified isolates that were absent from the Flavobacterium and Pseudomonas controls but present in the Aquaspirillum and Chromobacterium controls. However, no serologic cross-reactions were observed. Our results showed that the unclassified bacterium was distinct from any previously known genus of bacterium.