Metathesis, usually described in descriptive or historical linguistics as sporadic change, is investigated as a systematic phonological change using data from Gascon, an Occitan dialect. In the first chapter, the controversy over metathesis as a phonological change is presented and discussed from the standpoint of historical development. In contrast to the view that metathesis represents mainly a phonetic output, it is argued that metathesis can be seen as a phonological process following predictable patterns, as illustrated in different diachronic processes occurring in related dialects of Eskimo. In chapter 2, major historical and linguistic factors affecting development of the Gascon dialect are presented, and early analyses of one region's Gascon are examined. The third chapter introduces the data to be analyzed and the theoretical phonological framework for their examination. It is argued that the generative approach does not account adequately for the occurrence of this phenomenon in Gascon. The sociolinguistic approach to rules and one notational system are then used to arrive at the rule account of metathesis. In chapter 4, a discussion of assimilation in Gascon leads to the argument that because assimilation is no longer productive, metathesis occurs as an alternative, supporting the proposed rule account. (MSE)