Tetrahydrofuran (tetramethylene oxide) has been recommended as a histological dehydrating and clearing agent for use in human pathology by Dahme and Rothemund (1) and more recently by Haust (2). Dahme and Rothemund recommended its use after a search for a non-toxic reagent miscible with both water and molten paraffin wax. These authors state that the toxicity of tetrahydrofuran only approaches that of ethyl ether, whereas dioxane, with similar properties, is highly toxic. Tetrahydrofuran would appear to be as safe for routine laboratory use as toluene and xylene when similar precautions are observed. Haust (1958) gives various schedules for its use on human pathological material, and compares it with the more traditional methods. She concludes that it is simpler to use, and at least equally as efficient as the conventional dehydrating and clearing agents. In our work on insects we have extended its use to a solute for fixing solutions, and for dissolving celloidin in a double embedding technique.