Lygus antennal protein (LAP) is an olfactory-related protein of the tarnished plant bug Lygus lineolaris (Hemiptera, Heteroptera: Miridae), a hemimetabolous insect. In previous work, a polyclonal antiserum was generated against the N-terminal sequence of LAP; LAP immunoreactivity was strongest in antennae of adult males, but was also present in antennae of adult females and of nymphs. In the current study, LAP immunoreactivity was examined to determine the species specificity and the tissue and cellular localization of LAP expression. Western blot analysis indicated that LAP immunoreactivity was present in the antennae of the male congeners L. lineolaris and L. hesperous, but was not detectable in male antennae of the more distant relatives Podisus maculiventris or Nezara viridula (Hemiptera, Heteroptera: Pentatomidae). Western blot analysis further confirmed that LAP expression was restricted to antennal tissue. Histological analyses showed that LAP expression within the antennae was specifically associated with chemosensory sensilla on the antenna. Within the sensilla, LAP immunoreactivity was distributed throughout the extracellular lumen and was concentrated in dense granules within the cytoplasm of sensillar support cells. LAP immunoreactivity was restricted to a subset of antennal chemosensory sensilla, specifically the multiporous olfactory sensilla. These findings suggest that LAP has an important olfactory function in Lygus sp., possibly related to that of odorant-binding proteins (OBP) found in other insect orders. If so, LAP would be the first OBP-like protein characterized outside the Endopterygota.