This study demonstrates that short-term behavioral and physiological responses may permit the sailfin molly Poecilia latipinna to cope successfully with extreme hypoxia and suggests an interaction between behavioral response (aquatic surface respiration [ASR]) and physiological compensation. Poecilia latipinna acclimated to chronic hypoxia (6 wk at 1.0 mg L(-1) O(2)) exhibited higher hemoglobin and red blood cell concentrations and a 17%-19% lower critical oxygen tension than fish acclimated to normoxia. Ventilation frequency increased twofold under acclimation to hypoxia, a response that did not diminish with time. However, the use of ASR was an immediate response to hypoxia that decreased over the acclimation period. This suggests that gradual physiological compensation decreases the threshold for ASR. There was no consistent effect of hypoxia on mortality and no effect of hypoxia treatment on the number of gestating females, suggesting that plastic behavioral and physiological responses in P. latipinna compensate for hypoxia to a degree that mitigates a decrease in survivorship and facilitates continued reproduction in a laboratory setting. However, there may be predation costs in the field related to ASR.