Starting from an interpretation of the cleavage-theory as a dynamic model of "freezing" and "unfreezing" of party systems and voter alignments, this paper tries to ascertain the major cleavage structures that have conditioned the development of the Japanese party system. After its formation in 1955, the post-war party system was mainly based on two cleavages -one economic and the other a cultural cleavage -but we have seen a growing tendency towards "unfreezing" since the early 70s. Neither the

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... ly 70s. Neither the LDP nor the SDPJ, the two most important post-war parties, were able to transform themselves into catch-all parties, which led to a dramatic increase in the number of non-party supporters and floating voters. This dealignment brought about an instability of the system and a great fluidity in party labels in the 90s. The current system seems to be based on conflicts within the established elite rather than on social cleavages. The "unfreezing" of the system, and its further untying from the social structure, could cause a further weakening of its acceptance among the electorate.