If ecologism is neither conservative nor liberal, then is it socialist? Is it “green outside, red inside” as a watermelon, as liberals and conservatives claim? Ecologists were rather in this direction, in the 1970s. But the reception is quite cold. Socialists and communists consider ecological issues as a “secondary front” in a double sense: because they seem less pressing, less urgent, on the one hand, and because they can only be addressed by triumphing on the main front, that of class struggle. What strikes ecologists, however, is that class struggle fuels the capitalist development of productive forces. Socialists are therefore “productivist”. This main divergence breaks down into several others, which are closely linked: respective share of centralized (State) and decentralized, scientific and technical choices, way of life to promote and especially strategy of social movements on which each relies. The dispute ultimately concerns what “socialization” can mean.