Mesenchytraeus antaeus sp. n., an unpigmented giant enchytraeid inhabiting Carmanah valley (48°37′N, 124°44′W), Vancouver Island, is described. With its 102–127 segments and the size of fixed specimens reaching 61×2.9 mm (diameter maximal at midbody), this species qualifies as one of the largest members of the Clitellata family. The body wall comprises: a surprisingly thin cuticle (0.002 mm), the epidermis (0.02–0.04 mm), circular muscles (0.02 mm), plus thick longitudinal muscles (eight-layered, totally 0.16–0.18 mm) divided into seven unequal fields. The chaetae are grouped in bundles of two to seven but show some unusual traits which suggest interesting convergences with the lumbricids. For instance, the most lateral chaeta in a bundle is always the largest; the larger chaetae exceed in length one-third of a millimetre; the ectal tips of the chaetae are oriented in opposite directions in the anterior and posterior halves of the body. The spermathecal pores are prominent and open laterally in the middle of V, an unprecedented location among enchytraeids. The nature of the spermathecal ampullae (free, diverticulate and greatly elongate) indicates a close relationship to a group of species that seems to be confined to the Pacific coast of North America and the Arctic coast of Siberia. This amphi-Beringean lineage includes other gigantic forms such as M. harrimani Eisen, 1904 and M. maculatus Eisen, 1904, from which, however, M. antaeus is clearly differentiated by the lack of pigmentation, the different size distribution of the chaetae within bundles, and the unique arrangement of the male apparatus. The latter includes unusually long sperm funnels (extending over 10 segments) and two groups of large accessory glands opening through two circular papillae at the base of each penial bulb, independently of the penial pores. All other congeners known to possess this type of glands (e.g. M. franciscanus Eisen, 1904) are at most only half as large as M. antaeus. The chaetal muscles of M. antaeus are heavily infested by encysted nematodes.