Abstract Three hundred and twenty-five bacterial clones were isolated from the luminescent organs of 12 specimens of three species of the bathyal fish family Macrouridae ( Nezumia aequalis, N. stelgidolepis, and Sphagemacrurus hirundo) from both the Atlantic and Pacific oceans and four specimens of the midwater Opisthoproctus grimaldii. All of the isolates were the luminous psychrotrophic bacterium Photobacterium phosphoreum. This bacterium is the luminous species best suited to the cold temperatures of the deep-sea habitats where these fish families are encountered. On the other hand, previous studies have shown that luminous bacteria from luminescent organs of shallower water fishes of the temperate regions (Monocentridae) and tropics (Leiognathidae) contain the mesotrophic bacteria P. fischeri and P. leiognathi, respectively. The differences in luminous bacterial symbionts, the distant phylogenetic relationships, and the marked differences in the structure and location of the luminescent organ among these four fish families suggest an independent and multiple origin of each of these bacteria-fish associations.