Many epizootics in marine wildlife have been associated with increasing temperatures. Those pathogen outbreaks can cause severe decline in host populations which are experiencing strong selection by such infectious diseases. A relevant disease evoking global concern is the “Summer Mortality Syndrome (SMS)” leading to mass mortalities in Pacific oysters (Crassostrea gigas), which has been associated to high temperatures in conjunction with pathogenic bacteria, notably of the genus Vibrio. Within the European Wadden sea SMS has only been observed for southern populations. Since northern and southern populations are genetically distinct the question arises, whether a northward expansion of SMS is facilitated by temperature directly, or whether genetic differences between northern and southern populations might potentially constrain a further expansion. Until now the ecological and evolutionary implication of emerging marine diseases such as SMS still remains unknown. However, to completely understand an infectious disease the isolation, identification and characterization of respective causative agents is fundamental. We isolated various Vibrio sp. strains from Pacific oysters. by sequencing of 16srRNA and constructed a phyolgenetic tree. Based on 16srRNA phylogeny we singled out suitable candidates for controlled infection experiments on adult oysters to estimate taxonomic pathogen diversity under elevated summer temperatures. With this approach we are now able to derive predictions on the pathogenicity of opportunistic Vibrio sp. from their phylogentic position relative to known virulent strains. This comprehensive study on diversity and pathogenicity of Vibrio strains forms the basis for further experiments investigating differences in response to a selective force, such as SMS between southern and northern populations of the Pacific oyster within the European Waddensea.