The CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated genes) system provides prokaryotic cells with an adaptive and heritable immune response to foreign genetic elements such as viruses, plasmids, and transposons. It is present in the majority of Archaea and almost half of Bacteria species. Porphyromonas gingivalis is an important human pathogen, proven to be an etiological agent in periodontitis, and has been linked to systemic conditions like rheumatoid arthritis and cardiovascular disease. At least 95% of clinical strains of P. gingivalis encode CRISPR arrays, suggesting that they play an important function in vivo. Here, we show that all four CRISPR arrays present in the P. gingivalis W83 genome are transcribed. For one of the arrays, we demonstrate in vivo activity against double-stranded DNA constructs containing protospacer sequences accompanied on their 3’ end by NGG protospacer-adjacent motif (PAM). Most of the 44 spacers present in the genome of P. gingivalis W83 share no significant similarity with any known sequences, although four spacers are similar to sequences from bacteria found in the oral cavity and gastrointestinal tract. Four spacers match genomic sequences of the host, however none of these is flanked at its 3’ terminus by the appropriate PAM element.