A new method is proposed for the assessment of competition in clonal trials with completely randomized single-tree plots, without needing to choose a covariable a priori linked to competition. This method is applied to a coffee clonal trial planted in a completely randomized single-tree plot design located in Côte d'lvoire. Microenvironmental effects were first taken into account using the conventional Papadakis method. The competition effect (Ck) of each done (k) was then estimated as the influence of its presence on the residual value, after removal of the clonal effect, of its neighbors. Residuals were thus modeled as a linear regression of the neighboring clones' C value. Competition effects were shown this way to explain 4% of the residual yield in young trees and 10% in adult trees. Several clones had C values significantly different from 0. Some were identified as aggressive for their neighbors (C < 0), others as stimulating (i.e. promoting yield, C > 0). Architecture and vigor variables that were likely to be related to the C value of clones were then sought. For young trees, vigor, estimated by stem diameter, was best correlated to competition effects: vigorous clonal were more aggressive than others. When trees became adult, the length of the orthotropic internodes (Lort) was the trait that most effectively explained (42%) the competition effect of clones: the shorter the intemodes, the less aggressive was the done. In the future, Lort may thus be used in a selection index to prevent from selecting aggressive coffee doses that would undergo their own aggressiveness when grown alone in plantations.