The western yellowjacket wasp (Vespula pensylvanica) invaded Hawai`i’s national parks and refuges following its spread throughout the islands in the late 1970s. The endemic arthropod fauna of Hawai`i is thought to be especially vulnerable to these predacious social Hymenoptera, and methods of wasp control have been a priority for conservation biology in Hawai`i. The efficacy of the insecticide fipronil mixed with minced canned chicken meat for suppression of yellowjacket populations was evaluated in five experimental field trials in Hawai`i Volcanoes National Park between 1999 and 2005. Populations of Vespula were monitored in replicate twoto four- hectare study areas in mesic montane and seasonal submontane forests, before and after application of chicken bait, with and without 0.1% fipronil, and in treatment and nontreatment areas. The bait was applied in hanging bait stations for two to three days. The response of yellowjacket wasp populations was measured using at least three different metrics of abundance including instantaneous counts of wasps at bait stations, wasp traffic rates at Vespula nests, as well as heptyl butyrate trap and/or malaise trap catches in the study areas. All indices of wasp abundance exhibited significant reductions in sites treated with fipronil compared with non-treatment sites with the exception of malaise trapping, where only a limited number of traps were available to be deployed. Wasp traffic ceased at all Vespula nests in sites treated with fipronil within a month after baiting in four of the five trials. The only trial where fipronil failed to terminate yellowjacket nest activity occurred late in the fall when wasps switch from feeding on protein to carbohydrate foods. Based on these data, 0.1% fipronil in chicken bait appears to be an effective tool for suppressing local Vespula yellowjacket populations in the park and other natural areas during the period of peak wasp activity in the summer and early fall months.