The occurrence in Hotea gambia adults of a sexual dimorphism in the divided dorsal abdominal scent gland (dg 1)is reported. Counts made of ducted secretory units indicate that female dg 1 regresses at the end of larval development, unlike male dg 1 which undergoes no regression. Other dorsal abdominal scent glands (dg 2, dg 3) which function in the larvae cease to function during the imaginai moult. From gas chromatographic, mass spectrometric and [¹H]NMR data, the identity of the secretion from male adult dg 1 was established as virtually pure (E)-2-hexenol (a 100-mg mature male adult contains 0.5-1 μl of secretion). 2-Hexenol was also found in the reduced female adult dg 1. In the sexually monomorphic metathoracic scent gland, (E)-2-alkenals (C₆, C₈) and (E)-4-oxohex-2-enal, together with monoterpenes (β-pinene, limonene) but not 2-hexenol, were identified. The vapor of (E)-2-hexenol is repellent to both sexes of Hotea adults and toxic to blowfly (Calliphora) eggs.