Aerobic or 100% CO2 cultures of Aeromonas hydrophila growing in Tryptone Soya Broth at 5, 10 or 28 degrees C, were examined for filamentation and chain formation. Microscopy indicated that filamentation and chain formation occurred at ail temperatures in the presence of 100% CO2. No such filamentation was noted in aerobically grown cells. Cultures exhibiting filamentation were not proteolytically or haemolytically active. Reversion to normal morphology and enzymatic activity occurred within 24 h of subsequent aerobic incubation. The reasons for and adaptive value and pathogenic implications of this response are discussed. (C) 1998 Academic Press.