The 'string' test has been described as a test for the initial characterization of Vibrio spp., in particular, to differentiate between Vibrio and Aeromonas spp. Despite early literature questioning the validity of the test, its use has continued to be recommended. In this study, 49 Vibrio spp., 32 Aeromonas spp. and 47 enteric Gram-negative bacilli were tested and it was found that four genera gave a positive 'string' test, including Vibrio, Aeromonas, Pseudomonas and Klebsiella. The sensitivity and specificity of the test for separating Vibrio spp. from other Gram-negative bacilli were 86% and 70%, respectively. Hence the use of the string test in the initial characterization of Vibrio spp. should be supported by appropriate confirmatory tests.