A numerical taxonomic study has been carried out to establish the relationship of group F to other biochemically similar organisms within the family Vibrionaceae. A total of 154 strains were examined including 59 of group F. Out of 114 characters determined for each strain 100 were used to compute average Euclidean distances between strains. Four methods of clustering were used, all of which gave very similar results. Strains resembling Vibrio anguillarum fell into clusters corresponding to V. anguillarum, Beneckea nereida and a previously unrecognized group, phenon 5. Strains of the Aeromonas hydrophila/punctata group formed a heterogenous phenon within which certain subdivisions, perhaps artificial, could be discerned. Group F strains all fell in one closely‐knit cluster distinct from all the species of Vibrio, Aeromonas, Plesiomonas and Photobacteriwn studied. Group F strains could be divided into two biovars, I and II. Both biovars are present in aquatic, particularly estuarine, environments throughout the world but biovar I strains have also been isolated from humans with diarrhoea. It is concluded that group F is a synonym of group EF6 and that the strains within these groups should be classified in a new species named Vibrio fluvialis. The type strain is NCTC 11327.

A total of 19 of 20 (95%) strains of Aeromonas hydrophila biovar hydrophila and 16 of 17 (94%) strains of Aeromonas sobria isolated from a variety of clinical and environmental sources were found to be enterotoxin positive. Only 2 of 18 (11%) A. hydrophila biovar anaerogenes and 2 of 13 (15%) unidentified Aeromonas strains from a similar variety of sources produced enterotoxin. No association was apparent between the source of isolation, in particular diarrheal stools, and enterotoxigenicity; 41% of the isolates from diarrheal stools were enterotoxin negative. A strong correlation was noted between ability to produce enterotoxin and positive results in six characters: lysine decarboxylase and Voges-Proskauer reactions, production of gas from glucose, gluconate oxidation, xanthine hydrolysis, and hemolysis of human erythrocytes. In the majority of cases (35 of 39 strains), enterotoxigenicity was detected using cell-free filtrates of brain heart infusion broth cultures grown at 36°C for 15 h; however, the other four positive isolates were detected after growth in the same broth at 30°C or in Casamino Acids-yeast extract broth at 30 or 37°C. It is recommended that for enterotoxin tests, strains should be grown in both media at both temperatures. The infant mouse test was found to be a simple and reliable method for detection of the enterotoxin. The toxin proved to be heat labile and not neutralized by cholera antitoxin. Aeromonads are ubiquitous waterborne bacteria belonging to the Vibrionaceae. The taxonomy of Aeromonas spp. has been in a state of flux for some years, and only now is it becoming better understood. At present, the most frequently used classification of motile Aeromonas spp. is that of Popoff and Véron (29), who divided them into Aeromonas hydrophila biovar hydrophila, A. hydrophila biovar anaerogenes, and a new species, Aeromonas sobria. More recently, it has been proposed that A. hydrophila biovar anaerogenes should be renamed Aeromonas caviae (28), but this remains to be validated. Even now, many laboratories are unable to distinguish these species readily, and nonspecific references to A. hydrophila in reports until very recently can be interpreted as encompassing both biovars of A. hydrophila (sensu strictu) and A. sobria. These species, usually under the name A. hydrophila, have been isolated frequently from soil, foods, and a variety of animals, mostly those associated with water, such as fish, reptiles, and amphibians; in fact, these species have long been recognized as pathogenic agents causing hemorrhagic septicemias in these types of cold-blooded animals (12, 13, 17, 23). Their ability to cause opportunistic infections in compromised human hosts has been accepted for some time, and reports of various types of A. hydrophila infections, particularly in swimming-related injuries, in previously healthy persons are increasing. References to such reports are listed by other authors (6, 17, 32). Several investigators have reported the isolation of A.