The Alu family of repeated sequences from the human genome contains two distinct subfamilies. This division is based on different base preferences in a number of diagnostic sequence positions. One subfamily of the sequences, referred to as the Alu-J subfamily, is very similar to 7SL DNA in these positions. The other subfamily, Alu-S, can be divided further into well-defined branches of sequences. These findings revise the previous picture of the Alu family and expose their complex evolutionary dynamics. They reveal sequence variations of potential importance for the proliferation of Alu repeats and relate them to their structural features. In addition, they open the possibility of using different types of Alu sequences as natural markers for studying genetic rearrangements in the genome.