Male and female of Leptothorax wilsoni are described. The species apparently is workerless and parasitizes colonies of a species belonging to the Leptothorax "muscorum" group (sp. B). Nests of the new species were found on Mt. Monadnock, New Hampshire, USA. L. wilsoni has brachypterous females with a reduced thorax, a gynomorphic parasitic female from Mt. du Lac des Cygenes, Quebec, Canada, most probably belongs to the same species. L. wilsoni differs from its host species and other Leptothorax in the reduction of subapical mandibular teeth, the relatively short propodeal spines, and the distinctive foveate-reticulate sculpturing of the head, alitrunk, petiole and postpetiole.