The dependence of rigidity on task difficulty was examined. The hypothesis was that mentally retarded subjects would be more rigid than would nonretarded subjects on difficult but not on easy tasks. An intelligence test and seven rigidity tests varying in difficulty were administered to 45 retarded and 45 MA-matched nonretarded subjects. Factor analyses yielded one rigidity factor for the retarded group and two for the nonretarded group. In both groups the rigidity tests were interrelated and formed a Guttman simplex structure in terms of difficulty. The groups did not differ on three easy tests but did differ on four more difficult tests. The results were interpreted mainly in a motivational framework, with implications concerning mental retardation and rigidity as an interactional concept.