Bull. Eur. Ass. Fish Pathol., 25(2) 2005, 70 A novel O-serotype in Tenacibaculum maritimum strains isolated from cultured sole (Solea senegalensis) R. Avendaño-Herrera 1 , B. Magariños 1 , M.A. Moriñigo 2 , J.L. Romalde 1 and A.E. Toranzo 1 * 1 Departamento de Microbiología y Parasitología, Facultad de Biología, Universidad de Santiago, 15782, Santiago de Compostela, Spain; 2 Departamento de Microbiología, Facultad de Ciencias, Universidad de Málaga, 29071, Málaga, Spain. Abstract Tenacibaculum maritimum was consistently isolated from tenacibaculosis outbreaks affecting sole farms in Portugal and south of Spain in the last two years. These new T. maritimum isolates could not be assigned within the two major serotypes (O1 and O2) already described. Rabbit antiserum was prepared against one Portuguese sole isolate to examine the antigenic relationships between the isolates from sole using microtitre agglutination tests, dot blot assay and immunoblotting of lipopolysaccharides. Serological characterization of the recent sole isolates demonstrated that they belong to a novel O-serotype named O3, allowing us to extend the serological scheme for this fish pathogen. This information is useful for epizootiological and vaccination studies. *Corresponding author’s email: mpaetjlb@usc.es Tenacibaculosis or flexibacteriosis is recognized as an important infectious disease in marine fish since 1979 (Hikida et al., 1979; McVicar and White, 1979). The presence of the etiological agent Tenacibaculum maritimum (formerly Flexibacter maritimus) was first demonstrated by Bernardet et al. (1990) and since then, this pathogen has spread among a wide variety of host species, producing significant losses in cultured fish such as turbot ( Scophthalmus maximus ), Atlantic salmon ( Salmo salar ) and rainbow trout (Oncorhynchus kisutch), sea bass (Dicentrarchus labrax), gilthead sea bream (Sparus aurata) (Devesa et al., 1989; Pazos et al., 1993; Bernardet et al., 1994; Ostland et al., 1999) and, lately, in sole (Solea solea and S. senegalensis) (Cepeda and Santos, 2002; Avendaño-Herrera et al., 2004). In previous work we analysed the antigenic diversity of T. maritimum from sole, gilthead sea bream and turbot, which allowed us to propose a O-serotyping scheme for T. maritimum composed by two major serotypes (O1 and O2) mainly associated with the host species (Avendaño- Herrera et al., 2004). However, since the middle of 2003, new outbreaks of tenacibaculosis have occurred in sole farmed in Portugal and south of Spain. These new T. maritimum isolates could not be assigned within the serotypes already described. We report here the existence of a