Enterotoxigenic Escherichia coli (ETEC) strains of serotype O153:H45 have been found recently to be a frequent cause of sporadic cases and outbreaks of neonatal diarrhoea in Spain and the most important cause of infant diarrhoea in Chile. Relationships between sugar fermentation patterns, resistance to antibiotics and plasmid profiles were analysed in nine E. coli O153:H45 strains isolated in Spain that synthesised CFA/I antigen and STa enterotoxin. Derivative strains obtained by curing with acridine orange, and transconjugants rendered antibiotic resistant, were characterised phenotypically and analysed for plasmid content. Two fermentation patterns were recognised: rhamnose fermenters (four strains) and rhamnose non-fermenters (five strains). The ability to ferment rhamnose was the only differential characteristic found among 49 carbohydrate fermentation tests used to establish fermentation patterns. All nine strains possessed similar plasmid profiles of three or four plasmids of 52-87 Mda. A non-conjugative large plasmid of 82 Mda or 87 Mda, depending upon the strain, was identified as that responsible for production of both CFA/I and STa. Resistance to antibiotics was determined by plasmids other than those coding for CFA/I and STa. Two conjugative resistance factors were identified: a 52-Mda plasmid coding for resistance to ampicillin, streptomycin and sulphonamide in rhamnose-fermenting strains, and a 77-Mda plasmid coding for resistance to ampicillin, streptomycin, kanamycin, tetracycline and sulphonamide in rhamnose non-fermenting strains. Our results support the hypothesis that the prevalence and distribution of ETEC strains belonging to serotype O153:H45 in Spain and Chile could be due to the extensive cultural relations between Spain and South American from the past.