Chemical analysis of the venom of the myrmicine antMegalomyrmex foreli from Costa Rica revealed the presence of four major alkaloidal components. Two of these, 2-butyl-5-(E, 1-heptenyl)-5-pyrroline (3) and 2-butyl-5-(E, E, 1,3-heptadienyl)-5-pyrroline (4), constitute a new functional class of ant venom alkaloids, whose structures were assigned from their spectral and chemical behavior and unambiguous syntheses. The function of these compounds is suggested by field observations of the behavior ofM. foreli, its sting morphology, and the relative toxicity of 3 and 4 against termite workers.