Based on 28 criteria the taxonomy of 366 phage sensitive bacterial strains isolated from marine waters (Atlantic between European continental shelf and Sargasso Sea, Bay of Biscay, North Sea near Helgoland) was investigated. Seventy-eight phage-intensity strains derived from the same Atlantic Ocean regions as the sensitive ones were tested for comparison. While in the latter considerable diversity was observed, the results obtained with the phage-sensitive bacteria are characterized by stupendous uniformity. 362 of the 366 strains are assigned to the family Vibrionaceae, some 280 of which belong to the genusVibrio. As discussed, this taxonomic uniformity among the phage-sensitive bacteria is assumed to be an artifact mainly caused by the type of enrichment culture employed for the isolation of all but a few bacteriophage strains used and, to a lesser degree, by characteristics of the bacterial populations encountered.