Starting from Ishikawa’s hypothesis that irrigation is the ‘leading input’ in Asian rice agriculture, this paper examines the role of water control in Bangladesh’s agricultural development from 1949 to 1981. Water control–the provision of the right amount of water at the right time–is here defined to encompass drainage and flood control as well as irrigation. An analysis of inter-district variations in water control, fertilizer use, and the adoption of high-yielding varieties points to strong technical complementarities among these inputs. Water control differs from other inputs in that it generally requires prior capital investment and institutional arrangements for coordinated action among many producers. The combination of input complementarity and these special attributes suggests that water control may pose the key technological constraint to agricultural growth in Bangladesh. A district-level analysis of the relationship between water control and agricultural growth, crop yields and cropping pattern further confirms the central importance of water control in Bangladesh’s agriculture.