Haller''s organ in A. tridentatus consists of a capsule and an anterior group of sensilla. The capsule is the hollow in the cuticle on the dorsal surface of the 1st tarsus, where 4 pored hairs of olfactory sensilla are situated under the cover of the roof, formed by an anastomosis of the upper brunches of pleomorphs (capsule''s bottom non-sensory cuticular outgrowths). The canal of the accessory ampullaceous sensillum opens in a capsule near the bottom. The anterior group of sensilla consists of 2 parts: proximal part, containing pored grooved and thin hairs, is homologous to the anterior group of ixodid ticks, and the distal one which has no homologous in ixodids. Fine structure of all the sensilla in the mentioned parts of Haller''s organ is described in detail.