A field evaluation of a novel, 5% controlled-release formulation of pyriproxyfen was carried out in 400-500 liter concrete water storage jars, the most common and important larval habitat of the dengue vector, Aedes aegypti, in Cambodia. The formulation consisted of cylindrical resin strands, 3 mm in diameter and 40 mm in length. Pyriproxyfen was applied to 100 jars at a target dose of 0.03-0.04 mg of active ingredient (a.i.) per liter (30-40 ppb) in households in Phum Thmei, a village near the capital city of Phnom Penh, in April 2005. Inhibition of adult emergence (IE) in field populations ofAe. aegypti was measured every 2 weeks for 34 weeks. IE in treated jars exceeded 90% for 20 weeks, and remained above 80% until the end of the study. In 25 untreated jars, failure of pupae to metamorphose into viable adults remained below 4%. No alteration of taste or other undesirable effects of the treatment were reported by householders. This single treatment provided control of Ae. aegypti in water jars for the length of the main dengue transmission season in Cambodia that normally extends from May to November.