An investigation was made at the Mwea Irrigation Settlement, Kenya, to discover the reason why rice units which were transplanted earlier gave higher yields. Yields over an 11-year period were examined in relation to meteorological data and cultivation activities. It was found that the period during which the units were flooded prior to transplanting correlated with the date of transplanting, with longer preflooding periods causing higher yields and not the transplanting date. A yield/temperature relationship was also established.