Microbial populations associated with the amphipodBoeckosimus affiniswere examined using scanning electron microscopy and by detailed characterization of viable isolates. Bacterial populations were observed on food particles in the midgut, on the anal plates, and on faecal matter, but no bacteria were observed on the outer surfaces or on the gut lining of the amphipod. The dominant bacterial populations associated with the amphipods were in theVibrio–Beneckeagroup. Exposure to petroleum hydrocarbons resulted in a decreased dominance of the vibriolike bacterial populations associated with the amphipods. During captivity (without feeding) there were successional changes in the bacterial populations associated with the amphipods; the diversity of the bacterial community increased and the bacterial populations became less stringent in their physiological and nutritional requirements.