An insecticidal alkaloid, cocculolidine was extracted from fresh leaves of Cocculus trilobus DC. Von Braun reaction and a novel acid catalyzed degradation showed that this alkaloid had the same skeleton with those of erythrina alkaloids. Structure (I) was finally assigned to cocculolidine, being identified as a new lactone erythrina alkaloid containing α, β-unsaturated-γ-lactone. The mass spectra of I and dihydro-β-erythroidine were also discussed. © 1967 Taylor and Francis Group LLC.