Dietary Se supplementation boosts larval lepidopteran immunocompetence against per os baculoviral challenge. The mechanism of this Se-dependent immunostimulatory effect is currently unknown; however we hypothesize that selenoproteins present in various tissues of challenged larvae play a role in elevated resistance. We have determined the Se levels in plasma and various tissues of larval budworms. Larvae were fed known concentrations of the micronutrient Se in artificial diet, and the rate of Se assimilation into tissues was determined using instrumental neutron activation analysis. Additionally, larvae were subjected to dietary Se repletion and depletion by switching them between basal and Se-supplemented diets. At intervals following the onset of depletion or repletion larvae were bled to obtain plasma, and organs were dissected for analysis. Larvae fed diets containing 5-25 ppm Se exhibited elevated plasma concentrations of the micronutrient. An in vitro end point dilution assay demonstrated that concentrations of tissue Se were directly proportional to virucidal activity against AcMNPV present in the plasma, and to larval resistance. A discussion of larval Se sequestration and assimilation will be presented.