An examination is made of the applicability of quantitative and qualitative diatom analysis to the diagnosis of death by drowning, definition of the environment in which drowning occurred, and delimitation of the area where it occurred. the material comprises 107 bodies of subjects known or suspected to have died by drowning together with a control series of 15 bodies of subjects over 30 years of age who had died of various diseases on land. Whenever diatoms were found in the greater circulatory organs they were also found in the lungs, and when none were present in the lungs none were found in the other organs either. No diatoms or fragments of diatoms were found in the samples from the control subjects. All the fresh, well-preserved bodies for which death by drowning could be regarded as certain from the macroscopic autopsy findings and police reports, the cases used to test the method, gave quantitative diatom results that supported a diagnosis of water aspiration. The diatoms identified in the qualitative analyses served well to describe the ecological properties of the environments in which death had taken place, and the site of drowning could be defined by means of comparative water samples provided that sufficient diatoms were present, the local environment was not too homogeneous or the diatoms were not of quite different species due to a completely unknown location of death.