The alarm pheromone of southern California populations of the antForelius foetidus(Buckley) was identified as 2-heptanone. This compound (2.9µg per worker) occurs in an admixture withcis, trans-iridodialand a minor unidentified iridodial isomer, which together constitute a defensive secretion emitted posteriorly from the gaster. Secretory components were isolated and quantified from extracts by gas chromatography and structurally elucidated by1H and13C nuclear magnetic resonance analyses. Videorecordings of agonistic encounters with foreign ants revealed a stereotypical defense behavior ofF. foetidusworkers culminating in aerial transfer of their secretion onto the heterospecific worker. The defensive spray was repellent but not toxic to eitherSolenopsis maniosaorCrematogaster califomicaChemical and behavioral comparisons betweenF. foetidusandF. (=Iridomyrmex) pminosusindicate a high degree of similarity between these sympatric ants and support the generic reclassification of the latter species.