Abstract The partial sequence (600 bp) containing the most variable region of Vibrio vulnificus 16S ribosomal DNA (rDNA) was determined for 208 randomly selected V. vulnificus strains isolated from Galveston Bay, TX, USA between June 2000 and June 2001. A comparative analysis of the determined partial 16S rDNA sequences revealed the existence of two different partial 16S rDNA sequences (type A and type B, 1.3% base substitutions) among the 208 V. vulnificus isolates. A higher proportion of 16S rDNA type A strains was isolated in June and July while a considerably higher proportion of type B strains was isolated in September. In addition, after no V. vulnificus strains were detected during the winter months (December-February), only type A strains were isolated during the following months (March-May). The results suggest that the relative abundance of type A and type B V. vulnificus strains in Galveston Bay varies with the season and that the differences between the two 16S rDNA types may affect the viability of these organisms in the natural environment.