A rapid and manual mathematical method for comparing ecotoxicologic data has been investigated. It uses a classification procedure based on the calculation of chi 2 distances between the different elements of a data matrix. The classification is carried out using a simple graph-theoretical procedure (Kruskal's algorithm), allowing the construction of minimum spanning tree (MST). From an ecotoxicologic data base of 18 bacterial tests carried out on eight heavy metals the construction of the MST is explained in detail. Even if the results obtained are directly dependent on the data set chosen, they illustrate the heuristic potency of the minimum spanning tree method in comparing and estimating the dependence between environmental data.