A total of 63 Aeromonas strains isolated from diarrhoeal faeces or water samples were tested for adhesion to HEp-2 cells. An association between diarrhoea and high level adhesion was observed in that 12 of the 34 faecal isolates and none of the 29 environmental isolates yielded greater than 20 bacteria per HEp-2 cell in the adhesion assay. The proportion of high adherers was significantly greater for A. sobria (57%) than for A. hydrophila isolates (19%). Three of the eight faecal A. caviae isolates were also found to be high adherers. All of the environmental isolates were heavily pilated with pili having a mean diameter of 5 nm and a mean length of 420 nm; these were termed type-S pili. Of the 34 faecal isolates, 32% possessed S pili, and 68% were lightly pilated with up to 15 thin, flexible type-L pili, of mean diameter 2.5 nm and mean length 960 nm. Type-L pilation was associated with a high level of HEp-2 cell adhesion, and was more common in A. sobria and A. caviae than in A. hydrophila isolates. These results suggest that adherence to HEp-2 cells is a useful model for the investigation of Aeromonas enteropathogenicity, and that adhesion may be pilus-mediated in this organism.