The haemagglutinating ability of 141 isolates of Aeromonas spp. for human, horse and guinea-pig erythrocytes was examined. Although the majority of isolates (136/141) agglutinated human group O erythrocytes, all the eight possible patterns of agglutination were observed. Haemagglutination of human group O erythrocytes, but not horse or guinea-pig, was associated with the ability to agglutinate yeast cells (Saccharomyces) and with aggregation in a low concentration of ammonium sulphate. Haemagglutinating ability was further characterized by reactions in the presence of mannose, galactose or fucose. All the possible patterns of inhibition with individual sugars were observed, but haemagglutination of human group O erythrocytes not inhibited by mannose, galactose or fucose was more common among isolates from patients with diarrhoea, and isolates producing a Vero cell cytotoxin than would be expected by chance. This might represent a virulence factor. When used as a typing system haemagglutination patterns did not correspond with the clustering of isolates expected on epidemiological grounds. Repeated subculturing resulted in a loss of agglutinating ability, an increase in the number of sugars inhibiting haemagglutination and an increase in the salt concentration required for aggregation. Haemagglutination did not seem to be useful as a means of typing aeromonads.