Sequential pattern mining has been an emerging problem in data mining. In this paper, we propose a new algorithm for mining frequent sequences. It processes only one scan of the database thanks to an indexed structure associated to a bit map representation. Thus, it allows a fast data access and a compact storage in main memory. The experimental results show the efficiency of our method compared to existing algorithms. It has been tested on synthetic data and on real data containing sequences of activities of a urban population time-use survey.