The Sadgapas and the Tilis, two Bengali castes broke with their parent castes. They formed themselves into new castes which gained higher social status than their parent castes in terms of the local caste hierarchy in Bangal. The emergence of the Sadgopa caste, as distinct from the Tilis, occurred at a period when none of the technological, political, and intellectual developments had yet occurred in Bengal that are generally used to characterize modernization. They were established as a caste by the second decade of the nineteenth century while the history of their growth and development goes back to the second half of the sixteenth century. On the other hand, the Tili movement took an extensive form in the second half of the nineteenth century. The Tilis receives wider social recognition as a caste during the third and fourth decades of the twentieth century. The Tili movement was accelerated by modern conditions. Apparently the external factors helping social mobility varied from the case of the Sadgopas to that of the Tilis. But there are certain common features of development in both cases. Both the Sadgopas and the Tilis had collectively abandoned their traditional occupation to switch over to comparatively more lucrative and prestigious occupations, and became landowners. Complete dissociation from the traditional occupations which identified them with lower social ranks made it easier for the Sadgopas and the Tilis to aspire for better social status. But the crucial factor in their movements for mobility was ownership of land, which enabled them to have direct control over the life of the people in their respective areas and enhance their social prestige and power. This was the source of their strength as distinct groups and die source of their collective power to bargain successfully with the rest of the society for higher status. The incentive of corporate social mobility originated, both under traditional, pre-modern circumstances and under the circumstances of modernization, from the achievement of each group of a sense of corporate solidarity, regarding internal as well as external prestige. This enabled the groups to break away from the parent castes and to form new castes with higher social status. Previous writing on the subject has made this corporate solidarity a function of response to external forces, which are identified with only factors of modernization. It is the contention of this paper that corporate solidarity could have had its genesis in prcmodern times as well and that modernization marked only its acceleration.