The work reported here concerns the well known fact that the rice coleoptile elongates more rapidly and more extensively under water than in air. Decapitation experiment carried out with seedlings in air or under water, as well as experiment done with the growth response to the added IAA of floating or submerged coleoptile sections suggest that the superior growth of coleoptiles under water depends intimately on a decreased capacity of auxin it being destroyed under water. IAA disappears more rapidly from coleoptile tissue infiltrated with IAA and ihcubated in air than from similar coleoptile incubated under reduced oxygen. Experiments on the inhibition of growth by KCN as well as DNP suggest that the growth of rice coleoptile is determined by the two factors ; (1) decreased auxin destruction at low oxygen tension, and (2)decreased aerobic respiration at low oxygen tension.