As shown by PIMENTEL GOMES (1965), the theory proves that the use of the arithmetic mean of diameters to estimate basal areas in forestry leads to a bias. This paper evaluates this bias in the computation of cut out basal area in forestry thinnings, by means of theoretical study, samples generated in a computer, and also through the study of actual populations of trees in groves of Araucaria angustifolia (Bert.) O. Ktze, Pinus elliottii Eng., P. taeda L. and P. caribaea var. hondurensis Mor. The study thus carried out showed that the bias indicated can be rather serious.