Enteropathogenic antibiotic resistant E. coli (K99+, ST+) were selected and numbered in the faeces of cows and their newborn calves from a farm where outbreaks of neo-natal diarrhoea occurred occasionally. K99+ E. coli were resistant to nalidixic acid. This resistance rarely encountered, enabled to number selectively these bacteria. In the eight affected calves, the K99+ strains were simultaneously resistant to nalidixic acid, ampicillin, streptomycin, kanamycin, chloramphenicol, tetracyclines and sulphonamides. In the cases of severe diarrhoea, these K99+ E. coli predominated in the faecal flora. Strains presenting the same characters were also isolated from the faeces of the mothers, where they were generally found in lesser numbers. When calves were prophylactically treated with gentamicin or when their mothers had been vaccinated against enteropathogenic E. coli, clinical signs of diarrhoea did not appear or were mild, but carriage of K99+ E. coli persisted. These first results contribute to a better understanding of the epidemiology of neonatal colibacillosis in calves.