In the present study, we compared two enteroadherent Escherichia coli (EAEC) not belonging to the classical enteropathogenic O serotypes with respect to their attachment to and invasion of HeLa cells. Depending on the pattern of adherence to HeLa cells, one of the strains showed localized adherence (LA), and the other showed diffuse adherence (DA). Electronmicroscopic study showed that LA-EAEC produced the intimate attaching and effacing lesions and intracellular penetration in cultured HeLa cells. In contrast, DA-EAEC exhibited fimbrially-mediated adhesion to HeLa cells. Both LA and DA possessed morphologically distinct fimbriae. LA-EAEC expressed rod-like fimbriae, whereas fibrillar fimbriae were observed in DA strain. Ultrastructural study showed that the mechanisms of invasion by both the strains were different. LA-EAEC strains invaded the cells after pedestal formation and were enclosed in membrane-bound vacuoles. In contrast, DA-EAEC showed no membrane dissolution and pedestal formation during internalization of HeLa cells. A carefully controlled study will be necessary to establish the pathogenic role of DA-EAEC in diarrhoea.