After being a member of EELV and its precursor Les Verts for 20 years, François de Rugy announced his resignation on Thursday and, in a longstanding French political tradition, published a book explaining the move on the same day.
Called Ecology or ultra-leftism, you have to choose, it slams the Greens' shift to the left since they left the Socialist-led government in 2014.
De Rugy is angry at a recent decision to ally with Jean-Luc Mélenchon's Left Front, which includes the Communist Party, in some parts of the country in December's regional elections.
He is a longstanding partisan of alliances with the Socialists, an unpopular stance with the rank and file since the party saw its vote evaporate while it had two ministers in Jean-Marc Ayrault's government.
De Rugy worked with Ayrault on the local council of Nantes, western France, for several years and was part of the Greens' negotiating team with his successor as prime minister, Daniel Valls.
Since Euro-MP Daniel Cohn-Bendit left in 2012, another four MPs or senators have resigned from EELV, some because they found the party too left-wing, some because they found it too right-wing.
Others could follow, if press reports are to be believed.
The other leader of the parliamentary group, Barbara Pompili, and the party's leader in the Senate, Jean-Vincent Placé, both recently attended a meeting of Greens who support Hollande, which could be the basis of a new party of what de Rugy calls "reformist ecologists".
Placé recently threatened to lead a split from the part, like de Rugy accusing it of being "ultra-left".