Plant area index (PAI) was estimated in 3-year-old plantations of Eucalyptus globulus Labill. from (i) allometric relationships obtained from biomass harvesting, and (ii) canopy transmittance measurements. Diffuse light canopy transmittance was measured with a multi-cone sensor consisting of five inverted cones to scan a hemisphere. Plant area index was calculated from transmittance measurements by the inversion method assuming an ellipsoidal angle distribution of uniformly distributed canopy elements, or by application of the Beer-Lambert Law using an empirically determined extinction coefficient (k) for diffuse light of 0.5. Plant area index values of the experimental plantations varied as a consequence of diffferent water and mineral nutrient application rates. The PAI estimated from biomass harvesting ranged from 1.61 to 4.04. The estimates of PAI obtained by the ellipsoidal angle distribution method were significantly different from those based on the allometric relationships or from transmittance measurements assuming a value of k for diffuse light of 0.5.