Drought occurring mainly during the flowering period is responsible for a substantial decrease in the production in sunflower. The hypothesis that early sowing of sunflower would increase the probabilities of avoiding warm and dry period during the reproductive stage was tested. Nevertheless, early sowing is associated with low temperature du ring the first development stages. Phenological determinations were performed to study the effect of early sowing on the vegetative and reproductive periods. A sunflower population of recombinant inbred lines (RILs) was stud ied in two sites (France and Algeria) during 2007 at normal sowing date (control) and one-month earlier sowing. Phenostage observations were determined from emergence to harvest. Weather data with rain fall and temperatures were daily recorded. Cumulative growing degree day requirements fo r each phenological stage were calcu lated. Earliness of flowering was observed in the two sites when sunflower genotypes were sown one-month earlier in the season. Differences in thermal time requirement for sunflower development observed between sites and between early and control sowings could be explained by varia tions in base temperature values and/or photoperiod effect. A significant variability between genotypes was observed for sunflower development. The genotype ranking was not affected by early sowing for vegetative stage on the two sites, but during the post-flowering stage in Algeria, high temperatures and dry conditions occurring during this period considerably reduced their variability in the phenostage (R6-R9) and modified the genotype ranking. Genetic basis of sunflower phenostages response to early sowing must be explored in terms of genetic variability for temperature x photoperiod interactions.