The European slave-making antHarpagoxenus sublaevis has an unusual female polymorphism. In addition to ordinary workers, there are two kinds of reproductive females, the very rare full queens and the much more common ergatoid (i. e. workerlike) queens. A genetic mechanism is responsible for the development of larvae into either ergatoid or full queens. Sixty-one colonies of the closely related North American speciesHarpagotenus canadensis were collected from several localities in the provinces of Ontario and Quebec, Canada, and a total of 241 workers, 30 intermorphs, and 26 dealate full queens were dissected in order to discover whether a similar polymorphism occurs in this species. However,H. canadensis was found to have an essentialy normal polymorphism. All the fecundated reproductive females that we found were dealate full queens. A spermatheca, which constitutes the only significant difference between ergatoid queens and workers inH. sublaevis, is absent in all workers and intermorphs ofH. canadensis. In addition, we found that colonies ofH. canadensis are monogynous (likeH. sublaevis). However, minority of workers and intermorphs lay eggs even in queenright colonies. The average number of ovarioles is much lower inH. canadansis workers than in those ofH. sublaevis; but, in both species, it varies over about the same range (table IV). In this respect,H. canadensis workers resemble more closely the workers of the host species. The number of Malpighian tubules is five in both the reproductive females and workers. The poison gland is about the same size in bothHarpagoxenus species, but the Dufour's gland inH. canadensis is considerably smaller than inH. sublaevis. Thus, in several respects,H. canadensis bears a closer resemblance to the common host genusLeptothorax (subgenusMychothorax) thanH. sublaevis does.