Two types of fimbriae, designated aggregative adherence fimbria I and II (AAF/I and AAF/II) have been described in enteroaggregative Escherichia coli (EAEC) strains. These fimbriae mediate the aggregative pattern of adherence (AA) to epithelial cells. The genes encoding the structural subunit of each fimbria have been designated aggA and aafA, respectively. The prevalence of these genes was investigated in 155 faecal EAEC isolates that displayed AA to HeLa cells and were isolated from children in São Paulo, Brazil. Hybridisation assays with aggA and aafA sequences showed that 9.7% of these isolates carried aggA, 3.9% aafA, and none hybridised with both sequences. Of the 78 isolates displaying AA that reacted with the previously described EAEC probe (CVD432), 19.2% and 7.7% hybridised with the aggA and aafA probes, respectively. None of the 77 isolates displaying AA but lacking the EAEC probe sequence hybridised with either probe. These results clearly indicate that additional factors are involved in the AA phenotype in these EAEC strains.