Lack of moisture near the soil surface commonly delays sowing, reducing grain yields of Australian wheat (Triticum aestivum) crops. Deep sowing would allow growers to make use of soil moisture lying below the drying topsoil, but the short coleoptiles of semidwarf wheats reduce emergence when sowing at depths greater than 5 cm. Selection of longer, thicker coleoptiles would help in improving emergence in hard or crusted soils, or when deep sowing, yet little is known of genetic control of coleoptile size in wheat. A diallel mating design was generated from crosses between 12 Australian and overseas wheats, and assessed for coleoptile size at different temperatures (11, 15, 19, and 23°C). Repeatabilities for coleoptile diameter and length were moderate to high on an entry-mean basis (R2 = 0.48 and 0.77, respectively), reflecting large genotype and small genotype × temperature interaction variances. Genotypic variation among parents translated into large and significant (P < 0.01) differences among F1 progeny (94–142 mm and 1.56–1.84 mm for length and diameter, respectively). General (GCA) and specific combining ability (SCA), and reciprocal effects were significant (P < 0.01) for length and diameter. Baker’s GCA/SCA ratio was high (0.62–0.77) for coleoptile length but intermediate for diameter (0.38–0.64), indicating strong additive genetic control for length. Further, GCA effects and parental means were strongly correlated (r = 0.81–0.91, P < 0.01) indicating parent length to be a useful predictor of progeny performance. Coleoptile lengths for progeny derived from Rht8, Rht9, and Rht12 dwarfing gene donors were generally shorter (c. –7 to –13%) but were still an average 47% longer than coleoptiles of Rht-B1b and Rht-D1b controls. The genetic correlation for coleoptile length and diameter was small (rg = –0.25 ± 0.15n.s.) suggesting that the two traits are genetically independent. Development of wheats with longer, thicker coleoptiles should be readily achieved in selection among partially inbred families from crosses targetting improved establishment.