Abstract. Our aim is to develop methodology for recognition of Internet portal users on the basis of their behaviours. This is a classification task in which we have thousands values of decision attribute and objects are described by means of sequences of symbols. We develop feature selectors which make our task tractable. Since the behaviour usually does not distinguish users, we introduce user profiles which are clusters of indiscernible users and we construct classifiers which assign descriptions of user behaviour with user profiles. We also derive specific for our task quality measures.