It has long been known that a series of late-instar larvae of Choristoneura fumiferana (Clem.) may be distinguished from one of C. pinus Free. by colour differences of the head and prothoracic shield: specimens of the former usually have dark heads and light prothoracic shields; specimens of the latter usually have light heads and dark prothoracic shields. However, intensive study of the larvae, including numerous specimens of C. fumiferana from the spruce-balsam fir areas of Algonquin Park, Ontario, and of C. pinus from jack-pine areas at Normandale, Ontario, has shown that there are highly significant structural differences in the head; these differences serve to identify small series of either species and most single specimens.