The MCS/SEL/BAS program provides a method for group recognition, based on a criterion of homogeneity within the groups. The basic aim of this clustering method is not to 'force' data into a number of separate groups, as it allows the possibility that a given element in the data set can be assigned to more than one group. Moreover, a parsimonious path through the groups is sought by selecting groups on the basis of two suitably chosen, peak-ordered criteria. This selection continues until a covering of the data set is obtained (i.e., until each element in the data set is assigned to at least one group). Then relationships occurring among the set of selected groups are investigated by means of two coefficients, called overlapping and cohesion coefficient, respectively. The utility of this program has been demonstrated here in elaborating large sets of data derived from mating type interactions of ciliates, but it can be used also for analyzing data derived from a wide spectrum of compatibility phenomena exhibited by other living organisms. Algorithms of this program are written in BASIC and formulated in a conversational mode for processing on a Macintosh. A computer program (MCS/SEL/BAS) is available from G. Mancini upon request.