Photodynamic therapy (PDT) is a new approach to cancer treatment for a variety of malignant tumors. A new photosensitizer, 2,3-dihydro-5,15-di(3,5-dihydroxyphenyl)porphyrin (SIM01), had been evaluated for its genotoxic effects on glioma cells (C6). Comet assay had been used to evaluate the potential genotoxic effect induced by SIM01 on the C6 cells. When SIM01 had been shown to be a powerful sensitizer no DNA strand break was detected in the absence of light. SIM01 localized in cytoplasm but not in the nucleus of the tumors cells, which supported the finding of undetectable DNA damage under darkness and low photodynamic dose. Cell exposure to 20Jcm(-2) after an incubation time of 2h with 0, 0.25, 0.5, 2 or 4μgmL(-1) induced less than 25% of cell death but significant Tail Moment changes. If DNA damage intensity increased according to SIM01 doses under light exposure, importance of repair seemed to increase proportionally to PDT-induced damage. Positive controls consisted of doxorubicin-treated C6 cells this mutagen being known to induce genetic damage. Whatever the conditions used SIM01 appeared to be less deleterious than doxorubicin. As the comet assay can not give us the certitude that no mutation, photoadducts or oxidative damage had been developed under light exposure this point will have to be verified with another mutagenicity assay. SIM01 appears to be safe from a mutagenic point of view something of importance as tumors of small volume in patients with a long lifespan are at first indicated for PDT.