Presented is a criterion for monothetic classification of phytosociological entities on the basis of species presence. The classification criterion springs from the fact that every species, in algebraic terms, gives origin to two equivalence classes. One class includes entities with the species present, and the other class lacking it. The classificatory criterion is embedded in a procedure that establishes the coincidence of two classes according to maximal intersection. The methods employed include principal component analysis, PCA, and the technique of minimum spanning tree, MST, applied to a similarity matrix of species. The method is illustrated by example using data from Glauco-Puccinellietalia.