Tests carried out in vitro showed that rapid development and number of leaf roots were factors both linked with the early flowering of tobacco plants. The varieties having the most rapid root development also produced a more abundant root system. This property corresponded to earlier flowering in the field. Among the tobaccos resistant to Chalara elegans which have been tested, S 101 having polygenic heredity flowered early and this trait was fully transmitted to first generation hybrids. The P49 variety having N. debneyi heredity was very late flowering. Used as a genitor, the heterozygote showed a more satisfactory precocity.