When grown under controlled high temperature conditions United States sorghum cultivars exhibited depressed grain yield in comparison with yield at moderate temperatures. High temperatures (day/night 33/28°C) between germination and initiation resulted in low grain yield. High temperatures at initiation and during the early phases of panicle development did not reduce floret production. In plants exposed to high temperatures before initiation, high temperatures late in the panicle development stage induced floret abortion, and even moderate temperatures after anthesis were associated with embryo abortion. The adverse effect of high night temperatures on grain yield is likely to be associated with inadequate assimilate availability to support dark respiration.