The limited amount of memory is the major bottleneck in model checking tools and algorithms based on an explicit states enumeration. Thus, techniques which allow to represent the states efficiently are precious for these tools. We present in this work a novel approach which enables to store the state space in a compact way. Though it belongs to the family of explicit storage methods, we qualify it as semi-explicit since states may not be explicitly represented in the state space. Experiments show that very compact representations are obtained, with an acceptable increase of the run time.