Low dissolved oxygen (DO) had a significant effect on specific growth rate (G~S~), length increment (I~L~) and haematocrit (Hct) of the mummichog Fundulus heteroclitus. Regardless of access to the water surface, F. heteroclitus maintained high growth rates (G~S~ and I~L~) at DO concentrations as low as 3βmgβO~2~βl^β1^. With access to the water surface, both G~S~ and I~L~ of F. heteroclitus decreased by c. 60% at 1Β·0βmgβO~2~βl^β1^ compared to all higher DO treatments. When denied access to the water surface, a further decrease in G~S~ (c. 90%) and I~L~ (c. 75%) was observed at 1βmgβO~2~βl^β1^. There was no effect of dielβcycling DO (1β11βmgβO~2~βl^β1^) with or without surface access on G~S~, I~L~ or Hct of F. heteroclitus. Similar trends between G~S~ and faecal production across DO treatments suggest that decreased feeding contributed significantly to the observed decrease in growth rate. Haematocrit was significantly elevated at 1βmgβO~2~βl^β1^ for fish with and without access to the water surface. Increased Hct, however, was not sufficient to maintain high G~S~ or I~L~ at severely low DO. When permitted to respire in the surface layer, however, F. heteroclitus was capable of maintaining moderate growth rates at DO concentrations of 1βmgβO~2~βl^β1^ (c. 15% saturation). Although aquatic surface respiration (ASR) was not quantified in this study, F. heteroclitus routinely swam in contact with the water surface and performed ASR at DO concentrations β€3βmgβO~2~βl^β1^. No hypoxiaβrelated mortality was observed in any DO or surface access treatment for as long as 9 days. This study demonstrates that surface access, and thus potential for ASR, plays an important role in providing F. heteroclitus substantial independence of growth rate over a wide range of low DO conditions commonly encountered in shallow estuarine environments.