Résumé
A capture-mark-recapture (CMR) study in a hybrid zone between two species of barbel. Barbus barbus and Barbus meridionalis, in the Lergue (southern France) failed to reject the null hypothesis of no difference in survival among phenotypes. Power calculations indicated that very intensive fieldwork should be carried out if this hypothesis was to be investigated again. The study demonstrated, however, that there Acre marked differences in survival between upstream and downstream sites. It is suggested that there is an environmental gradient of selection along the hybrid zone (extrinsic component of natural selection).