The strategy of " union of the left " was negotiated, adapted, transformed in left-wing dominated municipalities by the coordinated action of élective officiais, candidates and militants of the French Socialist and Communiât parties. Starting from a detailed analysis of the intra-left power relations and the municipal stratégies of the two parties before the municipal élections of 1971 and 1977, the author identifies their varieties and their basic structures. He uses organizational analysis to attack the premises and generalizations derived, inter alia, from the Weberian tradition. Whether socialist or communist, the French local officiai is seldom willing to be only an implementation agent. In fact, he has enough power to be able to adapt, or even transgress, his party's Une. The local officiais rather unorthodox behavior is not, however, a rébellion. It is the outcome of a complex negotiation between a periphery which seeks to stress its autonomy and a centre which accepts that its Une must be adapted, provided its basic stakes are kept in mind.