Morphologies of calcareous benthic foraminifera found in poorly oxygenated deposits differ from those present in highly oxygenated deposits. On the basis of this observation, I infer that benthic foraminiferal test morphology can be used to extrapolate relative amounts of dissolved oxygen in Paleogene deep-sea bottom water. Calcareous benthic foraminifera from DSDP samples of Cenozoic age from the world oceans and New Zealand Paleogene samples are classified into three categories: aerobic, anaerobic and intermediate forms. The ratio of aerobic versus aerobic plus anaerobic forms is useful for estimates of global changes in the oxygen content of deep oceanic waters. These data indicate that low oxygen deep-sea conditions developed in the world oceans in early Eocene and late Oligocene times. These low-oxygen events occurred coincidentally with episodes of oceanic warming and sluggish deep water circulation. This coincidence suggests that the main cause of the low-oxygen events is low velocity deep-sea circulation in those time intervals.