Ensemble methods improve accuracy by combining the predictions of a set of different hypotheses. However, there is an important shortcoming associated with ensemble methods. Huge amounts of memory are required to store a set of multiple hypotheses. In this work, we have devised an ensemble method that partially solves this problem. The key point is that components share their common parts. We employ a multi-tree, which is a structure that can simultaneously contain an ensemble of decision trees but has the advantage that decision trees share some conditions. To construct this multi-tree, we define an algorithm based on a beam search with several extraction criteria and with several forgetting policies for the suspended nodes. Finally, we compare the behaviour of this ensemble method with some well-known methods for generating hypothesis ensembles.