Physiological lodging was studied under natural conditions in 9 flax varieties. These varieties showed different degrees of resistance to lodging : « Taiga », « Natasja » and « Fany » were lodging-resistant; « Hera », « Emeraude and « Tissandre were less resistant, but able to recover from lodging ; « Norfolk Earl », « Liral Prince » and « Liral Beattle » were lodging-susceptible. The experiments were set up in 2 different geographical locations, where flax is normally cultivated in France. In looking for mechanical causes of lodging, we examined in particular morphological indices of the stem : height, diameter, internode length, dry matter distribution along stem. These indices were scored during the whole biological cycle. Fiber measurements were made at harvest time. Lodging intensity was noted as lodging occurred. In our experimental conditions, it seems that the most representative lodging criterion is short basal-internode length ; this criterion is associated with a slow elongation rate during early growth. Another index is a low value for the fiber length : diameter ratio. These criteria were also most independent of cultural conditions.