Consensus techniques and the comparison of taxonomic trees. Syst. Zool. 21:390–397.—A new problem in the science of classification is presented, along with its solution. The problem is to combine the information in several taxonomic trees into a single tree. The solution is a computational method for computing a tree which represents only that information shared by all the rival trees. Such a method, called the consensus method, can be used to “compare” several rival tree representations or to compute a more stable tree from slightly perturbed variants of the original data. A method is defined and demonstrated for each of two different types of trees: rooted, fully labelled trees, and rooted trees with unlabelled internal nodes.