A cDNA encoding an octopamine (OA) receptor (BmOAR1) was isolated from the nerve tissue of silkworm (Bombyx mori) larvae. Comparison of amino acid sequences showed that BmOAR1 is highly identical to OA receptors isolated from Periplaneta americana (Pa oa1), Apis mellifera (AmOA1), and Drosophila melanogaster (OAMB or DmOA1A). BmOAR1 was stably expressed in HEK-293 cells. OA above 1 µm led to an increase in intracellular cyclic AMP concentration ([cAMP]i). The synthetic OA-receptor agonist demethylchlordimeform also elevated [cAMP]i to the same maximal level (≈ 5-fold over the basal level) as that induced by OA. However, other biogenic amines, tyramine and dopamine, and chlordimeform were without effects. The [cAMP]i level raised by OA was lowered by antagonists; the rank order of antagonist activity was chlorpromazine > mianserin = yohimbine. Cyproheptadine and metoclopramide had little effect. OA above 100 nm induced a transient or sustained increase in intracellular Ca2+ concentration ([Ca2+]i), depending on the concentration of OA. Sequence homology and functional analysis data indicate that BmOAR1 is an α-adrenergic-like OA receptor of B. mori.