A differentiation of E. coli rough strains is generally not well established in bacteriological urine diagnosis although these strains are relatively often isolated from urine samples of patients with urinary tract infections (see table 2). An exact characterization of rough strains seems to be necessary especially for the distinction between recurrent and reinfection during therapy and follow-up studies. The application of phage typing for characterization of E. coli rough strains isolated from urine is reported in the following paper, using a set of 13 phages (A-M) introduced by Marsik and Parisi (8). Out of 284 E. coli rough strains 166 (58.4 per cent) were found to be typable. Altogether 56 different phage patterns were observed. The most common phage patterns are presented in table 1. It is evident that GHK is the most frequent pattern occurring in 7 per cent of the strains investigated. The next most common patterns are M and BCDEIJLM (4.9 and 3.5 per cent, respectively). These, together with AM (3.2 per cent) and F (3.2 per cent) account for 21.8 per cent of all the rough strains tested. From the results it is concluded that phage typing may be a valuable tool for differentiation of E. coli rough strains.