Lichens are complex organisms living in a symbiotic relationship with fungi and algae have recently received special interest in cancer research. The cytotoxic activities of Usnea filipendula Stirt. lichen extract was investigated on colon cancer cell lines, HCT-15 and HT-29. Sulphorhodamine B and ATP cell viability tests were used to monitor cytotoxic activity. The mode of cell death (apoptosis/necrosis) was determined using caspase-cleaved cytokeratin 18 (M30), caspase-3/7 activity and fluorescence staining techniques that included, Annexin-V, Hoechst 33342 and propidium iodide. Usnea filipendula showed dose and time-dependent antiproliferative effect in HCT-15 and HT-29 cells. The IC50 values in HCT-15 and HT-29 cells were 17.92 and 41.87 mu g/ml, respectively. The extract induced apoptosis in both cell lines especially in HCT-15 cells in which caspase-3/7 activity was increased. Usnea filipendula was cytotoxic to colon cancer HCT-15 and HT-29 cell lines by inducing early or late apoptosis as evidenced by translocation of phosphatidylserine, pyknotic nuclei and nuclear condensation. Further studies would help to understand the full potential of Usnea filipendula as a novel anticancer therapy.