The aerial part of Aloysia gratissima(Gillies & Hook. ex Hook.) Tronc., "Usillo", is used as aromatic and medicinal. It is a shrub of up to 3 meters, distributed in North America, from the south of the USA to the north of Mexico, and in South America up to the 37th parallel. As in other native species, the demand is covered by harvesting in wild populations, which brings about the deterioration of the resource and lack of homogeneity of the harvested product. The variability was characterized in nine populations of A. gratissimavar. gratissima from the northeast of San Luis, Argentina, based on morphological and phytochemical characters. It was found that the species in the region presents considerable heterogeneity. Diversity was detected in the chemical characteristics of the essential oils analyzed and the prevalence of mono and sesquiterpenes was related to the olfactory identities identified. The main components were the sesquiterpene spatulenol and the monoterpene 1,8 cineole.