The northern fowl mite, Ornithonyssus sylviarum (Canestrini and Fanzago), is an obligatory hematophagous ectoparasite of poultry throughout the temperate world that causes extensive economic damage in the poultry industry (Furman, 1971. In Fundamentals of applied entomology, 2nd ed., R. E. Pfadt (ed.). Macmillan Co., New York, New York, pp. 589-610). Certain experiments with northern fowl mites were contemplated for which it would be necessary to separate living adults from immature mites and living males from females. The identification of the specific life stages and sexes of this pest is critical for detailed studies of its biology and physiology. However, satisfactory characters for determining the stages and sexes of this species in the living state have not been described in the literature nor were they available from several experienced acarologists. Both written descriptions and figures of the species have been published over the years that have added details to its first description by Canestrini and Fanzago (1877, Atti del R. Istituto Veneto 4: 69-208). These publications have all been based on carefully cleared, mounted, and preserved specimens that revealed morphological characters of taxonomic value but that were not readily visible in the living state. A recent detailed description with illustrations of the female, male, and protonymph was provided by Evans and Till (1966, Bulletin of the British Museum (Natural History), Zoology 14: 107-370). Earlier, Cameron (1938, Canadian Journal of Research 16: 230-254) gave a written description and drawing of the larva. However, the deutonymph has never been accurately figured or described. Living northern fowl mites were studied to overcome these deficiencies, and as a result, it is now possible to distinguish the stages and sexes of live mites without injuring them and with a high degree of reliability of stage and sex recognition. The founders of the colony of northern fowl mites were obtained from Dr. Joyce A. DeVaney (U.S. Department of Agriculture, College Station, Texas) in 1981. Mites were reared on chicks caged in sheet metal chambers provided with a barrier moat at the top (Chamberlain and Sikes, 1950, Journal of Parasitology 36: 461-465). Sometimes, protonymphs can be distinguished from females without magnification on the basis of size and shape, but this method is not reliable because the state of repletion varies greatly and because there is too much overlap with other stages. The magnification and reflected light of a dissecting microscope do not increase the efficiency. Protonymphs and adults are very active, and it is impossible to see any useful characters. Engorged females, however, are easily selected because they are the largest and most robust of the stages. A method was needed to narcotize the mites so that their movement could be arrested and, at the same time, lead to protraction of their chelicerae. Cold anesthesia did not achieve both of these goals. The chelicerae present important structural differences that are helpful in distinguishing the sexes and stages of slide-mounted specimens. It was thought that some chemicals that are used to slow the movements of Protozoa might do the same with northern fowl mites. These chemicals and a few others were tested at one or more concentrations: ammonium hydroxide, chloroform, ethyl alcohol, ethyl ether, magnesium sulfate, methyl alcohol, methyl cellulose, phosphate buffer, potassium hydroxide, and water. They were largely without effect on mites, or they were lethal. Ethyl alcohol was the most satisfactory material, and it was useful at concentrations of 30-70%. A 50% concentration was selected for use. The choice of 50% over 30% was arbitrary, but the former percentage evaporated more slowly and retained its droplet shape on a microscope slide more readily than 70%. A single mite that was suspected of being the desired stage or sex on the basis of size was se-