Essential oils extracted from twenty-two genera of Labiatae have been examined by gas-liquid chromatography. The results support the taxonomic suggestion that there are two major series of genera: the one (GROUP B, El-Gazzar & Watson 1970) oil-rich, the other appearing to be oil-poor. Records of susceptibility to Puccinia menthae Pers. are nearly confined to the oil-rich group, but within this assemblage certain minor groupings of important genera may be resistant. Attention is drawn to commercial possibilities among large GROUP B genera (e.g. Hyptis, Plectranthus; as yet exploited hardly or not at all) known to be oil-rich and possibly resistant to Rusts including Puccinia menthae.