The influence of ambient oxygen concentrations on sustained swimming effort (less than four body lengths (L.)/second, fish approximately 20 cm long) was tested in goldfish acclimated to two levels of ambient oxygen, 95 and 15% air saturation, and in rainbow trout acclimated to 95% air saturation only. The acclimations and tests were done at 20 °C in the case of goldfish and 15 °C in the case of rainbow trout. Experiments were done in Blažka's activity apparatus.The swimming effort of goldfish acclimated to air saturation declined from 3 L./sec at about 2 p.p.m. of ambient oxygen to 1 L./sec at about 1 p.p.m.The swimming effort of rainbow trout acclimated to air saturation declined sharply from 4 L./sec at about 2.5 p.p.m. to 1 L./sec at 2 p.p.m. of oxygen.Goldfish acclimated to 15% air saturation were tested in two separate groups, namely those left overnight in the activity chamber and those tested within 7 hours after they were handled; results from both these groups did not show any marked difference from each other or from those of goldfish acclimated to high oxygen.Since acclimation to low oxygen does not indicate any change in the swimming effort–ambient oxygen relation in goldfish, it is possible that the oxygen response studied is mediated through a behavioral mechanism which does not involve those physiological aspects of respiratory metabolism which are influenced by acclimation to low oxygen.