This experiment probed the mechanism of photoperiod sensitivity in male sterile lines. Materials included wild type Nongken 58, conditional male sterile line Nongken 58s, Nongken 58sr (a mutant of Nongken 58s that is male fertile under long-day conditions), and a transferred conditional male sterile line W6154s, an indica line which was obtained by a series of complicated crosses originally with Nongken 58s and Zhengshan 97. Seedlings were transplanted 25 d after sowing, and three photoperiodic regimes were imposed: ND, the natural daylength from May to October (13-15h); LD, 16 h day/ 8 h night; and SD, 10 h day/14 h night. Photoperiodic treatments were applied from transplanting (see figure). The parameters used to define the type of photoperiodic responses were days from sowing to heading of main tillers, leaf numbers, main panicle development stage, ratio of spikelet number to panicle length (RSL), and seed-setting rate (SSR). The experiment was repeated three times.