Boron (B) deficiency causes grain set in wheat to fail. A wide range of genotypic variation in the response to low B has been observed. Genotypes were screened in low B in soil and sand culture, and classified into five groups, namely, very sensitive, sensitive, moderately sensitive, moderately tolerant and tolerant. At very low levels of B, the very sensitive to sensitive genotypes were completely male sterile and set only a few or no grain, while the tolerant genotypes set grain normally. Natural outcrossing was detected in these male sterile plants when a tolerant genotype was growing nearby. Grain set by cross fertilisation was markedly enhanced by a B application directly on the ear of the male sterile plants. Three practical implications are suggested. Firstly, genotypes that are tolerant to low B can provide a solution for grain set failure caused by B deficiency. Secondly, the potential for outcrossing in male sterile B deficient wheat has to be taken into account in the maintenance of pure lines in low B soils even though wheat is normally self pollinated. Thirdly, a simple and novel method for hybridization is suggested, in which B deficiency is used as fertility selective medium and male sterile female parents and fertile male parents are provided by genotypic variation in the response to low B.