Very high densities of Aedes cantator larvae were observed all along the tidal zone of the Southeastern part of James Bay, Quebec. Thus, its distribution is greatly extended westwards; it was formerly known only on the Atlantic Coast, from Goose Bay to Virginia. The ecology of larval breeding sites of Ae. cantator are analyzed and discussed. The associated mosquito species are Ae. implicatus during the spring and Ae. dorsalis which follow in the same larval habitats during the summer. Two possible mechanisms of dispersal of Ae. cantator to James Bay are suggested.