Résumé : Two rotational bands have been found in the transfermium nuclei 251Md and 255Lr, the latter being the heaviest nucleus so far studied in-beam. Both are assigned to a [521]1/2-Nilsson state by comparison to theory. The experiments have been carried out in the Accelerator Laboratory of the University of Jyväskylä (JYFL), where the array of germanium detectors JUROGAM was used in conjunction with the recoil-seperator RITU and the focal-plane setup GREAT for gamma-spectroscopic studies.