ABSTRACT . – Environmentally-transmitted symbiotic bacteria must balance selective pressures of host specificity and the abiotic environment. Here, we investigate genomes of three strains of Vibrio fischeri isolated from three different squid species living in various temperature and salinity environments. These sequences were compared to others from symbiotic and freeliving vibrios for gene orthology, functional subsystem, selection, and phylogenetic analyses. A V. cf. campbellii bacterium isolated from seawater also was sequenced and provided an outgroup for the selection and phylogenetic analyses. This investigatory study provides the basis for further directed studies that may elucidate the genetic architecture underlying adaptation to both the free-living environment and host squids in V. fischeri. Symbiotic vibrios in this study had relatively conserved genomes with evidence of strong purifying selection and few duplication events. Horizontal gene transfer may provide a mechanism for the acquisition of host and environment-specific genes. Despite spending a majority of time in the free-living stage and a low probability of host-colonization for any one bacterium, symbiotic V. fischeri appear to have evolutionary histories molded by their squid hosts.