Summary — The relationships between quantitative variations of 2 flavanonols in Scots pine needles and Diprion pini larvae mortality were studied. Those 2 compounds were characterized as taxifolin (T) and its glucoside (TG) after hydrolysis and analysis by TLC, HPLC and spectrophotometry. Quantitative differences between 30 clones were more important for TG than for T, nevertheless clones which presented a content of taxifolin higher than 1.5 mg g -1 DW showed a T/TG ratio equal to or greater than 0.5 (fig 2). Quantitative changes were also observed throughout the year. The amount of taxifolin peaked in autumn as those of its glucoside decreased (fig 3). Darkness also induced a gradual increase of T but no significant effect on TG (fig 4). Storage of twigs during feeding tests and insect defoliation both induced a strong glucosilation of taxifolin in needles (table I). High rates of mortality of Diprion pini larvae were associated with the presence of T and TG both in needles and faeces ( (fig 3). De même, l'effet de l'obscurité sur les rameaux provoque une