The presence of the enteroaggregative Escherichia coli (EAggEC) heat-stable enterotoxin 1 (EAST1) gene was investigated in 15 strains each of EAggEC, enteropathogenic E. coli (EPEC), EPEC-related strains of non-EPEC serotypes, diffusely adhering E. coli (type 1 DAEC) that carries F1845 adhesive pili (or a related adhesin), and enteroinvasive E. coli (EIEC) by PCR and colony hybridization. The EAST1 gene or its homologue was present in 53.3% of EAggEC, 20% of EPEC, 13.3% of the EPEC-related strains, and 6.7% of type 1 DAEC, EIEC and E. coli unrelated with diarrhea had no gene with sequence similarity to the EAST1 gene. Comparison of the EAST1 gene sequences analyzed in this study as well as those reported previously showed that EAggEC (including strain O42, which was shown to be pathogenic in volunteer experiments), EPEC, type 1 DAEC, type 2 DAEC (which carries the 57-kDa outer membrane protein as an adhesin), and enterotoxigenic E. coli shared a common sequence. A variant type of the EAST1 gene sequence was present in the EAggEC strain 17-2 (initially characterized for the EAST1 gene) and in an EPEC-related strain of a non-EPEC serotype. These data suggest that the EAST1 gene or its variant is a virulence gene widely distributed among diarrhea-associated E. coli.