Yield losses due to boron deficiency are becoming common in the warmer subtropical regions into which wheat has been expanding. This study evaluated the B response of two wheat (Triticum aestivum L.) genotypes (SW41 and Sonora 64) in Chiang Mai, Thailand. A sand culture experiment imposed six B levels in nutrient solution (0, 0.1, 0.2, 0.5, 1, and 10 micromolar B). A series of field experiments in a sandy loam (Typic Tropaqualf) under rainfed conditions imposed four levels of borax application (0, 0.5, 1, and 2 kg B ha-1) over three successive years. In sand culture, B deficiency depressed the number of grains per spikelet in both genotypes (more strongly in SW41 than in Sonora 64). At 0 micromolar B, SW41 had no grain in any spikelet. In the field, B deficiency depressed grain number and grain yield of SW41 but not Sonora 64. Flag leaf B at boot stage of both genotypes grown in sand culture was closely related to the number of grains per spikelet at maturity; the critical level was determined to be 3 mg B kg-1. For field-grown plants, the data suggest that, at the boot stage, flag leaf B concentrations of 5 mg kg-1 are deficient, and 7 are sufficient. Boron deficiency can depress wheat yield through grain set failure, but susceptibility to B deficiency varies with genotype. Sensitivity to B deficiency should be included as a criterion for breeding programs for low-B soils