In recent analyses of the class structure of Britain and other modern western societies, arguments have been advanced relating mobility patterns to class formation and modes of class action. Three specific theses-those of `closure' at the higher levels of the class structure, of a `buffer-zone' around the division between manual and non-manual occupations, and of `counter-balance' in rates of inter- and intragenerational mobility-are critically examined on the basis of a survey inquiry into occupational mobility carried out in England and Wales in 1972. The sources of the various discrepancies which emerge between the three theses and the results of the enquiry are discussed, and major importance is attached in this respect to the evolution of the occupational division of labour in Britain over recent decades.