Aims This study describes a novel species within the genus Edwardsiella based on phenotypic and genetic characterization of fish pathogenic Edwardsiella isolates previously identified as E.tarda. Methods and Results Phenotypic characterization, DNA-DNA hybridization and phylogenetic analysis of representative Edwardsiella isolates from fish previously identified as E.tarda were conducted and compared with E.tarda type strain (ATCC 15947T). Phenotypically, strains from fish grow with pin-point colonies producing slight -haemolysis under the colony. In contrast to the E.tarda type strain, fish strains did not grow at 42 degrees C or degrade -methyl-d-glucoside (with the exception of NCIMB 2034), citric acid and l-proline. With the exception of strain ETK01, all fish strains were highly pathogenic to zebra fish, while ATCC 15947T and NCIMB 2034 were nonpathogenic. DNA-DNA hybridization (DDH) levels between representative fish isolates and the E.tarda type strain ranged from 15 to 43 center dot 6%, while NCIMB 2034 hybridised with the type strain at the level of 63 center dot 2%. DDH values between the various fish isolates ranged from 68 center dot 2 to 93 center dot 9% defining a new and separate DNA hybridization group differing from the E.tarda type strain consistent with the findings of phylogenetic analysis, in which the fish isolates comprised a separate clade. Conclusions Phenotypical and genetic characterizations demonstrated that Edwardsiella isolates from fish described in this study do not belong to the species E.tarda or any of the previously established taxa within the genus Edwardsiella. The fish related strains studied here (excluding NCIMB 2034) represent, therefore, a novel species within the genus Edwardsiella for which we propose the name Edwardsiella piscicidasp.nov, with strain ET883T (NCIMB 14824T=CCUG 62929) as the type strain. Significance and Impact of the study The current finding will improve the diagnosis, understanding of the epidemiology and in establishment of effective control measures against this serious fish pathogen.