A blood culture bottle from a patient with bacteremia contained both Proteus rettgeri biogroup 5 and Providencia stuartii (described in Bergey's Manual of Determinative Bactiology [8th ed., 1974] as Proteus inconstans), which had the same unusual antibiotic resistance pattern. Single colonies of this P. rettgeri biogroup 5 isolate were shown to produce urea- clones. If current taxonomy is used, the strain changed from P. rettgeri to P. stuartii in the laboratory and probably also in the patient. Urea- clones were also found in three of six other strains of P. rettgeri biogroup 5. No urea-negative clones were found in two isolates each of P. rettgeri biogroups 1 and 3. Previous data from deoxyribonucleic acid-deoxyribonucleic acid hybridization, biochemical reactions, and serological cross-reactions all have indicated that the taxon now called P. rettgeri biogroup 5 should be classified as P. stuartii urea+. We propose that this taxonomic change be made. Urease production is probably plasmid mediated in P. stuartii urea+ and can easily be lost, as shown in our case report and in three stock cultures. Urea hydrolysis will no longer be the key test for differentiating P. rettgeri from P. stuartii. Rather, acid production from trehalose, D-arabitol, adonitol, and D-mannitol will be the key tests. Whereas P. rettgeri is usually trehalose-, D-arabitol+, adonitol+, and D-mannitol+, P. stuartii has the opposite reactions.