This article argues that Gerd-Rainer Horn's model of a ‘Mediterranean New Left’ encompassing both the French Parti socialiste unifié (PSU, 1960–1990) and the Italian Partito socialista italiano di unità proletaria (PSIUP, 1964–1972) needs to be significantly revised. It agrees that, half a century on from the events which gave rise to their foundation, this much misunderstood part of the political spectrum, midway between social democracy and the far left, is worthy of rescue from the ‘enormous condescension of posterity’, but questions how similar the two parties actually were. Major differences emerge, especially in the nature of each party's relationship with communism, with the philosovietism of the PSIUP contrasting with the PSU's evolution towards an anti-Leninist decentralist socialism of self-management. Yet, at the same time, important new evidence is uncovered about the concrete political and personal links that developed between leading intellectuals of the PSIUP and PSU, an example being the friendship of the Italian parliamentarian and theorist Lelio Basso with the journalist Gilles Martinet, later French ambassador to Italy. Other transnational links, both across the Mediterranean and to eastern Europe, are explored. Furthermore, the location of the roots of both parties in the 1940s generation of anti-fascist resistance calls into question prevailing assumptions equating the New Left with the youth of the 1960s, with wider implications for our understanding of the development of the European left across the twentieth century.