Abstract: This study was carried out to evaluate the effect of temperature during the stage of flowering and seed maturity and the effect of genotype x environment interaction on seed yield and its components in recently developed varieties of berseem (Trifolium alexandrinum L.). Five varieties (Giza 15, Giza 6, Assiut population, Sakha 3 and Serw 1) were sown on three sowing dates (1st October, 1st November and 1st December) in a split plots in randomized complete block design. Three cuts were taken after (75, 115, 150 days from sowing). The experiment was conducted during 2006/2007 and 2007/2008 seasons. Results revealed that the highest number of florets (55.9) and seeds/head (40.9), seed set, (77.8%), 1000-seed weight (3.38 g) and seed yield (1.26 t ha-1) were obtained from sowing on the first of October and seed production taking place during the period from April to the end of May. The Giza 15 variety out yielded (1.10 t ha-1) other tested varieties followed by Giza 6 (1.07 t ha-1) and Assiut population (0.99 t ha-1) over both seasons. The estimates of phenotypic stability parameters (bi and S2di) for seed yield showed that the highest seed yielding varieties Giza 15 and Giza 6 exhibited less instability in seed yield while the Assiut population was more stable.