SYNOPSIS. The symbiotic association between the Hawaiian sepiolid squid Euprymna scolopes and the marine luminous bacterium Vibrio fischeri is being developed as a model system for the study of animal-bacterial interactions during development. Changes in light organ morphology during embryogenesis foster successful infection of the light organ with the proper bacterial partner. These embryonic events of light organ morphogenesis include the elaboration of an epithelial surface with a complex ciliated, microvillous field. The squid host hatches without the bacterial symbionts, but acquires them within hours from the free-living population of the bacteria in the water column. Upon exposure to the proper symbionts, the host organ undergoes a series of morphogenetic changes, including loss of the ciliated, microvillous field. The light organ then goes on to mature into a morphological configuration that serves to promotethe maintenance of a stable association with the bacteria and that correlates with the use of the bacterial bioluminescence in behavior of the host. This symbiosis is discussed in light of other cyclically transmitted animal-bacterial associations.