The diversity of microorganisms found in the marine environment reflects the immense size, range of physical conditions and energy sources, and evolutionary age of the sea. Because associations with living animal tissue are an important and ancient part of the ecology of many microorganisms, it is not surprising that the study of marine symbioses (including both cooperative and pathogenic interactions) has produced numerous discoveries of biotechnological and biomedical significance. The association between the bioluminescent bacterium Vibrio fischeri and the sepiolid squid Euprymna scolopes has emerged as a productive model system for the investigation of the mechanisms by which cooperative bacteria initiate colonization of specific host tissues. The results of the last decade of research on this system have begun to reveal surprising similarities between this association and the pathogenic associations of disease-causing Vibrio species, including those of interest to human health and aquaculture. Studies of the biochemical and molecular events underlying the development of the squid-vibrio symbiosis can be expected to continue to increase our understanding of the factors controlling both benign and pathogenic bacterial associations.