Changes in germination and potential seed longevity (Ki of the viability equation) were monitored during the development of watermelon (Citrullus lanatus) seeds grown in warm (minimum and maximum temperatures were 16.8 and 40.1°C) and cool (9.8 and 35.1°C) sites in 2002. Maximum seed weight (mass maturity, end of seed filling phase) was attained 25 days after anthesis (DAA) under both growing conditions. However, maximum germination was observed at 30 DAA in both sites. Potential seed longevity was achieved at 45 and 40 DAA, which were 20 and 15 days after mass maturity (the time of maximum seed dry weight) in warm and cool sites, respectively. When the maximum quality was achieved, fruit flesh was pale red, red, and 98–100% of the seeds were brown coloured. Seed dry mass and maximum potential longevity were consistently greater in cool than in warm growing sites. Dried seeds had higher germination percentages than those of fresh ones until mass maturity (25 DAA) at both sites. © 2004 Taylor & Francis Group, LLC.