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Heat tolerance is a desirable trait for wheat in the sub-tropics and in traditional wheat areas under threat from global warming, where the crop may also be exposed to boron (B) deficiency. The 13th High Temperature Wheat Yield Trial (13HTWYT) from CIMMYT was evaluated with Fang 60, a 1977-bred variety as a B-efficient check, in a sand culture with (+B) and without (-B) added B in the nutrient solution in Chiang Mai, Thailand where temperature for the coolest month and during the time of heading both averaged 22.3 degrees C. The wheat genotypes were affected differently in their grain set and grain yield by B deficiency (G x B, p < 0.001). Responses to B of the 13HTWYT expressed as grain yield in -B relative to + B correlated positively with the grain set index and the relative number of grains/spike, but not with the relative number spikes, while profuse tillering in -B was associated with lower yield. In -B grain yield also increased with the number of grains/spike. When B was not limiting, the grain yield increased with days to heading, number of tillers/plant, spikes/plant, spikelets/spike, grains/spike and grains/spikelet. Based on their grain set in -B, 39% of the 13HTWYT were considered B-inefficient, with grain set in fewer than half of competent florets, while 26% were B-efficient in the same order as Fang 60, 16% moderately efficient and 18% moderately inefficient. Boron efficiency is available in the 13HTWYT, suggesting that joint tolerance for B efficiency and heat stress can be selected if required. (C) 2019 Friends Science Publishers