Biomass estimation equations for individual holm oak (Quercus ilex spp. ilex) were developed using data from 71 sample trees. Diameter alone was a strong predictor of biomass, but better biomass estimates were obtained when height and crown projection radius were added to the model. In addition to total above-ground dry weight, different biomass components were calculated: stem (greater than 5 cm in diameter), branches 1 to 5 cm in diameter, branches less than 1 cm in diameter, and foliage. Grouping the trees according to height (taller or shorter than 7 m) and aspect (north-and south-facing slopes) resulted in significantly different allometric equations for most of the components considered. Improved estimates were achieved in many cases when D50 (diameter at 0.5 m) was used instead of DBH. Comparison of the results with other published equations developed for Q.ilex and other oaks showed differences, probably due to morphological diversity of trees and different DBH class distribution of samples used. © 1988 Oxford University Press.