We have developed a sensitive and specific polymerase chain reaction (PCR) method for identifying phytoplankton in cases of death by drowning, and we have designed four primer pairs, EG1, EG2, SK1 and SK2, for chlorophyll-related genes of Euglena gracilis and Skeletonema costatum, which are commonly distributed in all types of water. In order to evaluate the usefulness of this method for diagnosis of drowning, we have used this method for detection of plankton genes in non-drowned rabbits submerged after death and in decomposed drowned rabbits. Plankton DNA was identified in lung samples obtained from the non-drowned rabbits because of postmortem plankton penetration into the respiratory system, and plankton DNA was identified in liver and kidney samples obtained from the decomposed drown rabbits. The results show that our new PCR method is a useful method for diagnosing drowning.