With the API 20 E Enterobacteriaceae system of biochemical testing, a biotype, coded numerically, was determined for each of 574 strains of Escherichia coli isolated from patients with urinary tract infection. The serotypes of the strains were also determined. Fifty-five different biotypes were identified, two accounting together for 42% of the strains examined and seven others each accounting for between 8.4 and 1.9%. There was little correlation between biotype and serotype. Fifty pairs of strains were isolated from patients before treatment. In 43 the biotype and serotype of both strains of each pair were the same. In six pairs the biotype, but not the serotypes, differed, the difference being limited to the results of the tests for lysine decarboxylase. The biotypes of the strains of the remaining pair differed widely although their serotypes were the same. It is suggested that this method of biotyping offers a simple but accurate way of discriminating between recrudescent urinary tract infection caused by E. coli and that due to reinfection.