Under controlled environment conditions a semi-dwarf wheat, Mexico 120, was subjected to day temperatures of 32 and 27°C, with night temperatures of 21 or 16°C, which are similar to those occurring for a large part of the dry (winter) season of the Ord River valley area of northern Australia. Temperature differences imposed only during the 23 days between sowing and floral initiation produced effects which persisted throughout later growth, when all plants were grown under one temperature regime. The effects of temperature differences before floral initiation were greater than those of similar temperature differences imposed after ear emergence began. The maximum temperatures occurring even in the coolest months probably limit maximum potential grain yields in an environment similar to that of the Ord River valley.