Deepwater rice can grow in the regions of Southeast Asia that are flooded during the monsoon season because it has several adaptations allowing it to survive under flooded conditions. One such adaptation is the ability for rapid internode elongation upon partial submergence to maintain its foliage above the rising flood water levels. Ethylene is considered to be the trigger of this growth response because deepwater conditions not only trap ethylene in submerged organs, but also enhance the activity of 1-aminocyclopropane-1-carboxylate (ACC) synthase. Herein we have studied the expression characteristics of two members of the five-member multigene family encoding ACC synthase in rice OS-ACS1 and OS-ACS2 and show that partial submergence induces expression of OS-ACS1 and suppresses expression of OS-ACS2. The induction of OS-ACS1 occurs within 12 h of partial submergence and at low oxygen concentrations. The data also suggest that deepwater conditions posttranscriptionally regulate ACC synthase activity. OS-ACS1 gene expression may contribute to longer-term ethylene production, but not to the initial, growth-promoting increase in ethylene synthesis.