So-called 'cleaner fish', including various wrasse (Labridae) species, have become increasingly popular in Norwegian salmon farming in recent years for biocontrol of the salmon louse Lepeophtheirus salmonis. Cleaner fish mortalities in salmon farms are, however, often high. Various bacterial agents are frequently associated with episodes of increased cleaner fish mortality, and Vibrio tapetis is regularly cultured from diseased wrasse. In the present study, we investigated the genetic relationships among 54 V. tapetis isolates (34 from wrasse species) by multilocus sequence analysis (MLSA; rpoD, ftsZ, pyrH, rpoA and atpA). In the resulting phylogenetic tree, all wrasse isolates belonged to sub-clusters within V. tapetis subsp. tapetis. Slide agglutination testing further confirmed the complete dominance amongst these isolates of 4 O-antigen serotypes, designated here as V. tapetis subsp. tapetis serotypes O1, O3, O4 and O5, respectively. A pilot challenge trial using serotypes O3, O4 and O5 did not indicate high pathogenicity towards ballan wrasse Labrus bergylta, thus questioning the role of V. tapetis as a primary pathogen of this fish species.