Guanosine 3′,5′-monophosphate (cyclic GMP) was found in the accessory gland of reproductively mature male house crickets (Acheta domesticus (L.)) up to the exceptionally high level of 500 pmol/mg protein (190-4) mol/kg wet weight). The identity of cricket cyclic GMP was confirmed by enzymatic and spectral analysis. A survey of 10 closely related species of Orthoptera indicated that high levels of cyclic GMP in the accessory gland occur only in the subfamily Gryllinae, to which A. domesticus belongs. In these crickets GMP in the accessory gland increases together with protein content during two weeks after the final molt. Levels are not augmented by dissection, and are independent of the presence of sperm in the seminal vesicles and of the production of spermatophores by the gland. The function of cyclic GMP in the accessory gland is not yet understood.