A feeding inhibitor and a new insecticidal alkaloid were found in the leaves of Cocculus trilobus DC. One of the feeding inhibitory constituents in this plant, present at a concentration of about 0.007% on a fresh weight basis, was isolated and identified as the aporphine alkaloid, isoboldine. The threshold concentrations of inhibitory activity against Trimeresia miranda Butler and Prodenia litura Fabricus were about 200 p.p.m. for both insects. The insecticidal constituent, present at a concentration of about 0.03 % on a fresh weight basis, was isolated and identified as cocculolidine, a recently described erythrina alkaloid containing a five-membered lactone. However, these constituents are inactive against an insect which feeds on C. trilobus, Oraesia excavata Butler, suggesting an interesting aspect of the host-insect interrelationship. © 1968, American Chemical Society. All rights reserved.