Males of Galerucella calmariensis and G. pusilla (Coleoptera: Chrysomelidae) emit a pheromonally active compound while feeding on host foliage. Isolation of the compound from collected volatiles was guided by male/female comparisons of gas chromatograms and by GC-EAD. It was identified by mass spectrometry (EI, CI, and HR), NMR and UV spectroscopy, and microchemical tests as a novel dimethylfuran lactone. The structure was proven by synthesis. The synthetic compound was attractive in the field to the males and females of both species. The pheromone could become a tool for monitoring populations of these important biocontrol agents of purple loosestrife. A new method is described for distinguishing the two species, based on examination of the tibial spurs of the males.