Abstract— The survival of the planarian Dugesia dorotocephala was measured following treatment with germicidal ultraviolet (UV) radiation, with the DNA photosensitizer 8‐methoxypsoralen (8‐MOP) plus near UV radiation, or with the cytoplasm photosensitizer erythrosin plus visible radiation. The typical morphological response of an irradiated animal was lysis, with the head being the most sensitive part. All the survival curves had substantial shoulders. Treatments whose primary target was likely to be DNA (UV irradiation or 8‐MOP plus near UV irradiation) resulted in lysis of part of the planarian population which commenced 24 h or more after treatment and continued over several days. The treatment whose primary target was likely to be the membranes of the planarian (erythrosin plus visible irradiation) resulted in lysis mostly during the first few hours after treatment. Thus an easily discerned endpoint, lysis, can be used for D. dorotocephala to evaluate photosensitizing compounds, where the kinetics of lysis may differentiate among compounds whose modes of action are different.