The prevalence of enterotoxigenic E coli (ETEC) as a pathogenic agent of diarrhoea in the Tel-Aviv (Israel) area was determined, and the isolated E. coli strains characterized. During three periods (summer 1977, summer 1978, and summer 1979), a total of 335 specimens were tested for the presence of E. coli producing LT and ST toxin. Most of the specimens were from sporadic ambulatory diarrhoea cases (children and adults) attending a number of health care clinics in Tel-Aviv. Two to five colonies were tested from each sample. ETEC was detected in 69 cases (20%): LT/ST strains were isolated from 9 cases (2.7%); LT from 7 cases (2.1%); and ST from 53 cases (15.2%). ETEC was isolated in all age groups. In 19 specimens, 2 or more of 4 colonies tested were enterotoxigenic and were identical according to biotype, antibiotic sensitivity, and serogroup. These findings suggest that enterotoxigenic strains predominated in the bacterial population of the stool specimen. Part of the isolated ETEC strains belonged to serotypes already known as enterotoxigenic in different geographic areas of the world. The most frequently encountered were serogroups O8 (9 cases) represented by at least three serotypes, among them O8:K40:H9, and serotype O6:K15:H16 (5 cases); a number of serotypes were represented only by two cases or by single cases. Among 16 LT-producing stains (LT/ST and LT-only), 13 belonged to 3 serogroups, while ST-only strains represented a large spectrum of serotypes, some of which are now known as enterotoxigenic. Several serotypes common in other geographical locations were not detected.