Clustered regularly interspaced short palindromic repeats (CRISPR) consist of highly conserved direct repeats interspersed with variable spacer sequences. They can protect bacteria against invasion by foreign DNA elements. The genome sequence of Streptococcus mutans strain UA159 contains two CRISPR loci, designated CRISPR1 and CRISPR2. The aims of this study were to analyse the organization of CRISPR in further S. mutans strains and to investigate the importance of CRISPR in acquired immunity to M102-like phages. The sequences of CRISPR1 and CRISPR2 arrays were determined for 29 S. mutans strains from different persons. More than half of the CRISPR1 spacers and about 35 % of the CRISPR2 spacers showed sequence similarity with the genome sequence of M102, a virulent siphophage specific for S. mutans. Although only a few spacers matched the phage sequence completely, most of the mismatches had no effect on the amino acid sequences of the phage-encoded proteins. The results suggest that S. mutans is often attacked by M102-like bacteriophages, and that its acquisition of novel phage-derived CRISPR sequences goes along with the presence of S. mutans phages in the environment. Analysis of CRISPR1 of M102-resistant mutants of S. mutans OMZ 381 showed that some of them had acquired novel spacers, and the sequences of all but one of these matched the phage M102 genome sequence. This suggests that the acquisition of the spacers contributed to the resistance against phage infection. However, since not all resistant mutants had new spacers, and since the removal of the CRISPR1 array in one of the mutants and in wild-type strains did not lead to loss of resistance to infection by M102, the acquisition of resistance must be based on further elements as well.