COOL NIGHT TEMPERATURES 10 to 16 days before heading cause many rice florets to be sterile. The direct cause of sterility is failure of pollen grains to germinate because they are immature, and contain little if any starch. In two successive years, 12.5 and 12.8% of the florets in rice fields grown for certification were found to be sterile, as determined by random samples of panicles taken from fields. The range in sterility in 59 fields checked in 1972 was from 2.7% to 34.8%. Rice growers have recognized the problem for many years and have correctly associated it with late planting, cold water at the inlet boxes, and heavy nitrogen fertilizer applications.