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Adult Sitona cylindricoIlis Fåhraeus responded to adenosine in bioassays for feeding-stimulant activity. The molar concentration of adenosine which elicited maximum feeding in the bioassay was less than 1/10 of the sucrose concentration required for maximum feeding. Various purines, pyrimidines, nucleosides, nucleotides, and related compounds were compared with adenosine in bioassays for weevil-feeding stimulant activity. Of all compounds tested, only adenosine triphosphate and adenosine monophosphate appeared to act as feeding stimulants; at the concentrations used, their activities were substantially lower than that of adenosine. Free adenosine was isolated from leaf extracts of weevil-susceptible Melilotus officinalis (L.) Lam. by ion-exchange and paper chromatography and was identified by spectrophotometric and paper-chromatographic techniques. The quantity of adenosine present, ca. 12 μg/leaflet, was sufficient to indicate that this compound probably influences feeding of the weevil on M. officinalis plants.