Volatile components of Pachycoris stallii scent gland secretions and the bug's host plant, Croton californicus, were identified by gas chromatography and mass spectroscopy. The predominant compounds isolated from C. californicus fruit and leaves were beta-myrcene and beta-caryophyllene. Metathoracic gland secretions of P. stallii contained mostly (E)-2-hexenal, (E)-4-oxo-2-hexenal, (E)-2-hexenyl acetate, and n-tridecane. In males, n-tridecane was present throughout the metathoracic gland, but in females this compound was found only in the median reservoir/accessory gland. (E)-2-Hexenal was present throughout the gland of female bugs, but in males was primarily present in the median reservoir/accessory gland. (E)-4-Oxo-2-hexenal and n-dodecane were isolated from the median reservoir/accessory gland of male and female bugs. Metathoracic glands were sexually monomorphic. Data from chemical analyses and anatomical observations suggest that dorsal abdominal glands of adults were apparently obsolescent. In nymphs, dorsal abdominal glands produced (E)-2-hexenal, (E )-4-oxo-2-hexenal, n-dodecane, n-tridecane, and tetradecanal. The proportion of the predominant constituent, (E)-4-oxo-2-hexenal, decreased from 72% in the first instar to 47% in the fourth instar. Proportions of tetradecanal and n-tridecane were greater in the fourth instar than in the first instar. Observations of dissected glands indicated that median and posterior dorsal abdominal glands of all nymphal instars were more developed than anterior dorsal abdominal glands. Scanning electron micrography revealed the presence of polygonal microsculpturing on the integument surrounding the ostioles of metathoracic and dorsal abdominal glands. Chemical, anatomical, and behavioral data indicated that P. stallii has a chemical defense system based on short-chain carbonyl compounds and that this system is directed against arthropods. The abundance of arthropod natural enemies apparently has forced P. stallii to maintain this defense system despite feeding on a toxic host plant.