The current study was conducted to develop a cost-effective and reliable method to estimate population densities of 1st-instar bagworms, Oiketicus kirbyi (Guilding), in Costa Rican oil palm plantations. Total numbers of larvae per tree were correlated with numbers on 80, 160, 240, or all (320-360) folioles of all palm leaves. The r 2 values of these correlations were used to assess the accuracy of population estimates obtained by sampling leaves in different positions within the palm crown or different numbers of folioles per leaf. Larvae were most abundant on apical and subapical folioles of leaves in the upper palm crown. When a limited number of folioles per leaf (80, 160, or 240) was sampled, leaves in the upper crown provided the most accurate estimates of larval densities per tree. Decreasing accuracy of population estimates from upper to lower leaves resulted from increasing variability of larval proportions from upper to lower leaves. Larval populations of 2 other oil palm defoliators, Opsiphanes cassina (Felder), and Stenoma cecropia (Meyrick), are currently assessed by sampling leaf 17. Sampling O. kirbyi larvae on 160 folioles of leaf 17, in the middle palm crown, may represent an acceptable compromise among cost-efficiency, reliability, and simultaneous assessments of several defoliating caterpillars. Reasonably accurate estimates of larval densities per area obtained in this study by sampling 1 palm per hectare should not be interpreted as a general recommendation for sample size, because the effect of population density on optimal sample size per hectare remains yet to be investigated.