Seeds of watermelon (Citrullus lanatus) with a moisture content of 7.2% were heat treated at 60°C for 15 or 20 h, and their performances compared with that of untreated seeds. Radicle emergence of treated seeds was only slightly lower than that of the control when germinated at 25°C. To ascertain the effect of the treatments on hypocotyl growth (emergence) per se, radicle emergence was synchronized in all treatments and emergence from deep plantings in sand determined. Small differences in emergence were observed when seeds were germinated at 25°C, but seed treatment resulted in large reductions in emergence at 15°C. This demonstrates that the hypocotyls developing from heat-treated, germinated seeds are less able to withstand low germination temperatures, which could be an important factor in emergence failure. Levels of free abscisic acid (ABA) were higher at 15°C than at 25°C, with hypocotyls of treated seeds containing more free ABA and its breakdown products than the control at the lower temperature.