Terpenoid constituents have been isolated from the following Iridomyrmex ants. The known methylheptenone (11) and iridodial, a novel dialdehyde (C10H16O2), have been extracted from Iridomyrmex detectus (F.Sm.) and from I. conifer ((Forel). I. nitidus (Mayr.) has yielded iridolactone (C10H16O2). Iridodial is converted into iridolactone by the action of alkali. The isolation of iridomyrmecin (a lactone, C10H16O2) from I. humilis (Mayr.) confirms the original observation of Pavan (1949).