The mussel Alathyria jacksoni typically occurs in the main rivers of the Murray-Darling Basin and not in slow-flowing or standing waters. Conversely, Velesunio ambiguus is typical of billabongs and creeks and rarely occurs in big rivers except in the vicinity of impoundments. The exclusion of V. ambiguus from the main rivers probably reflects its weak anchorage, whereas the exclusion of A. jacksoni from billabongs may reflect its inability to withstand low levels of oxygen (hypoxia). At normal oxygen levels the mean rates of oxygen consumption for A. jacksoni and V. ambiguus are 21.5 (s.d. = 6.43, n = 17) and 13.5 (s.d. = 5.85, n = 10) �mol g-0.75 (dry tissue) h-1, respectively (a body-mass exponent of 0.75 was included in calculations of the mass-specific oxygen consumption rate). When exposed to declining oxygen, A. jacksoni shows little or no regulatory ability, whereas V. ambiguus maintains a steady rate of oxygen consumption down to 65 torr. Under extreme hypoxia, A. jacksoni loses muscular tone but V. ambiguus closes its values and remains inactive. On removal from water A. jacksoni incurs a significant oxygen debt after 10 days, but V. ambiguus shows no oxygen debt after 15 days. In general, animals adapted to lotic environments may need more oxygen, and may be more susceptible to hypoxia, than those adapted to lentic environments.