A provisional typing schema based on sensitivity to 23 bacteriophages has been established for Proteus mirabilis. Seventy-three bacteriophages were isolated on strains of P. mirabilis (64), P. vulgaris (1), P. morganii (7), and P. rettgeri (1), but those isolated on P. mirabilis were the most useful in differentiating other strains of . mirabilis. From the 73 phages studied, the best 23 were chosen by computer analysis for the provisional system, which was then used to study P. mirabilis infections in a 500-bed general hospital. All patient isolates for 19 months were saved and then compared by bacteriophage typing and the Dienes reaction in a retrospective study. There was evidence for only three instances of cross-infection or -colonization during this time. Bacteriophage typing was very sensitive in differentiating strains, since 200 strains were differentiated into 113 different lysis patterns and 94% were typable. The Dienes reaction was useful at times but often gave reactions that were difficult to read or that changed when the tests were repeated. The bacteriophages described by Schmidt and Jeffries were also evaluated and proved useful in combination with ours. The value of bacteriophage typing was clearly established, and work toward a standardized schema for P. mirabilis should continue.