The study included 138 A. sobria and 182 A. hydrophila isolated in Perth from samples of diarrhoeal or non-diarrhoeal faeces or from domestic water. Strains were grouped in relation to agglutination of human, horse, rat and guinea pig erythrocytes and the effect of sugars on haemagglutination. Agglutination of red cells of all four species (primary group 1) was most commonly associated with A. sobria, particularly those strains isolated from faeces of patients with diarrhoea. Most A. hydrophila associated with diarrhoea also belonged to group 1 but A. hydrophila from non-diarrhoeal stools or from water most commonly agglutinated human and guinea pig cells but not horse erythrocytes (primary groups 2 and 3). Fucose-resistant haemagglutination (FRHA) of strains in primary group 1 occurred with about 68% of the strains of A. sobria associated with diarrhoea. Mannose-resistant haemagglutination (MRHA) was limited to 29% of strains of A. sobria associated with diarrhoea. The predominance of primary group 1 among strains of Aeromonas spp. associated with diarrhoea and the proportion of these strains showing FRHA suggest that haemagglutination of cells from human, horse, rat and guinea pig, particularly if fucose-resistant, should be considered in a search for characteristics, which possibly contribute to virulence of Aeromonas spp.