A plant recently introduced into North America as the citrosa, Pelargonium citrosum ('Van Leenii'), has been marketed as a biological repellent against mosquitoes. Citrosa is claimed to repel mosquitoes within a 10 ft.2 (0.93 m2) area due to a continuous fragrant release of citronella oil. The total essential oil yield was 0.2 +/- 0.1% from fresh plant material. Chemical analysis by the authors revealed that combined essential oils of fresh greenhouse- and field-grown citrosa have 35.4 +/- 6.2% geraniol, 10.4 +/- 1.6% citronellol, 8.9 +/- 2.0% isomenthone, and 6.8 +/- 3.8% linalool. Both the morphology and essential oil of citrosa fall within the Pelargonium x asperum hybrid complex and are similar to 'Rosé', the commercial rose geranium. No character of morphology or essential oil of a Cymbopogon species yielding commercial citronella oil could be detected in the citrosa. The effectiveness of the citrosa as a repellent against field populations of spring Aedes spp. mosquitoes was evaluated and compared with a 75% deet (N,N-diethyl-3-methylbenzamide) formulation. Deet provided > 90% reduction in mosquitoes biting subjects for up to 8 h post-treatment. There was no significant difference between citrosa-treated and nontreated subjects.