The authors report investigations carried out with a view to improving the method for the detection of diatomic plankters for the purpose of forensic medical expertise of death by drowning. The methods for studying diatomic plankton present in blood, pericardial fluid, and contents of the sphenoid bone sinus are proposed to be employed for the purpose of differential diagnostics of death by drowning. Specific approaches to sampling cadaveric materials, their preparation and examination by dark field and phase contrast microscopy are described. It was shown that the application of the proposed methods for the investigation of diatomic plankton in cadaveric biological fluids ensures preservation of all planktonic species and thereby significantly improves the quality of analysis and decreases the time needed to perform forensic medical expertise.