The ocular Harderian gland (HG) in three Balaenopterid species (common minke whale Balaenoptera acutorostrata, sei whale B. borealis and Bryde's whale B. edeni) was investigated at macroscopic and microscopic levels. The gland encircled the eyeball similar to a belt form on mainly the corneal side, and was more developed in the rostral and ventral aspects of the eyeball. The multiple secretory ducts of the gland opened into conjunctival fornix and palpebral conjunctiva. The gland also possessed a prominent duct system composed of tubuli which were divided into lobes. Histochemically, the secretory cells of the gland were characterized by the presence of both acid and neutral glycosaminoglycans for Alcian blue (pH 2.5), periodic acid-Schiff, and the absence of lipids for Sudan black B. These characteristics indicated that there are differences from those of HG in some terrestrial mammals, and suggested it may be one of the adaptations for protecting eyes from sea water in aquatic environments.