Two microsatellite tandem repeated tetramers, (GACA) 4 and (CAAT) 4 , were used for Proteus mirabilis strain differentiation. The microsatellite-based PCR tests were applied for the examination of interstrain diversity for 87 P. mirabilis strains. Forty-six of the investigated strains were clinical isolates (5 were hospital isolates and 39 were outpatient clinic isolates); 42 strains were derived from the Kauffmann-Perch collection of laboratory strains. Fingerprinting done with the tetramers had a high discrimination ability [0.992 and 0.940 for (GACA) 4 and (CAAT) 4 , respectively]. The distributions of clinical isolates among well-defined laboratory strains, determined by numerical analysis (unweighted pair-group method with arithmetic averages; Dice similarity coefficient), proved their genetic similarity to reference strains in the Kauffmann-Perch collection. This analysis also indicated that it is possible to estimate some phenotypic properties of P. mirabilis clinical isolates solely on the basis of microsatellite fingerprinting.Proteus spp. are mobile gram-negative bacteria common in both the natural environment and animal or human intestinal tracts. Proteus spp. are also known etiologic agents for meningitis and numerous bacteremias (8,(20)(21)(22)(23)43). Urinary tract infections are among the most frequent bacterial infections (19), and Proteus mirabilis strains are one of the most common causes of urinary tract infections (7%), third after Escherichia coli (52%) and Enterococcus spp.(12%) (11). Such infections occur commonly among patients with structural defects of the urinary tract (6,38,39). The presence of P. mirabilis rods within a urease-induced bladder stone matrix was visualized recently (24). Moreover, some results suggest a possible etiopathogenic role of P. mirabilis in rheumatoid arthritis (9, 31), and some nosocomial transmission events have been reported (31). Because of the increasing spread and clinical significance of P. mirabilis rods (13,15,30,31,32), studies of effective methods for epidemiological investigations are of great importance.Out of the numerous types of simple sequence repeats proposed as tools for very sensitive bacterial fingerprinting (25,27,48,50,51,54), many microsatellites have been described as being useful for microbial differentiation, especially below the level of species (1,10,26,28,33,34,47,48,49,53).Most of the molecular fingerprinting methods applied for the differentiation of Proteus (35,36,44), however, are not sensitive enough for more detailed interstrain differentiation. In particular, no specific method allowing for P. mirabilis differentiation, especially below the serotype level, has been described so far.In this study, we have focused on microsatellite-based methods supplying patterns specific for particular P. mirabilis strains. The aim of the study was to verify how effective microsatellites are for P. mirabilis fingerprinting; in particular, we examined whether tandem tetramer-based PCR is applicable to Proteus strain differentiation or typing...