Cockerels artificially selected for low levels of plasma corticosterone response to social stress and maintained in a low social interaction environment rapidly developed large populations of northern fowl mites. Cockerels selected for high levels of plasma corticosterone response to stress and housed in a high social stress environment developed smaller populations of mites. Pullets were most resistant to infestation. Inherited levels of corticosterone had more effect on populations of this mite than did social stress alone. In this experiment, social stress did not significantly modify mite population development on male birds of the 2 steroid lines. The northern fowl mite, Ornithonyssus sylviarum (Canestrini and Fanzago 1877), is a blood-feeding ectoparasite of poultry. It is capable of completing its entire life cycle on the host. Roosters tend to maintain higher populations of mites than hens (Maw et al., 1935; Abasa, 1965; Matthysse et al., 1974). Matthysse et al. (1974) found a strain of White Leghorn hens selected for a high dietary requirement for arginine to be relatively resistant to the northern fowl mite. They also noted that chickens can develop circulating, precipitating, and skin-sensitizing (reaginlike) antibodies to this parasite. Another facet of this study indicated debeaked chickens to support greater mite populations than chickens with normal beaks. Payne (1930) observed that some chickens in a heavily infested flock may remain free of mites. Loomis et al. (1970) noted the presence of mite-free birds in heavily infested flocks. They theorized an immune response or hormone activity might be responsible. Social stress and steroids affect the resistance of chickens against several infectious diseases (Colmano and Gross, 1971; Gross, 1972, 1974; Gross and Siegel, 1975). High levels of steroids have been shown in some cases to stimulate phagocytic response (Vernon-Roberts, 1969). High levels of social interaction, selective breeding for high levels of plasma corticosterone, injection of ACTH, or the injection of corticosterone increase resistance of chickens to Escherichia coli septicemia (Gross and Colmano, 1970, 1971). High levels of steroids or social stress increase resistance to E. coli septicemia, Staphylococcus aureus infection, Streptococcus fecalis infection (Gross and Colmano, 1967, 1969), Trichinella spiralis (Davis and Reed, 1958), and Plasmodium berghei (Friedman et al., 1969). Lodmell et al. (1970) noted excessive populations of Polyplax serrata (Burmeister) on amputee mice subjected to a continually disturbed hierarchy. They concluded this was because of reduction in mutual grooming. Nelson (1962), working with sheep, suggested that any stressor sufficiently acute and continuous to cause an increase in corticosteroid secretion can effect major changes in the normal cyclic population curves of Melophagus ovinus (L.). Injection of ACTH, cortisone, or oral administration of acetylsalicylic acid was shown to decrease resistance of sheep to infestation with this insect. This report concerns the effect on development of populations of the northern fowl mite on chickens artificially selected for high or low levels of plasma corticosterone response to stress and housed to promote high or low levels of social interaction. These factors have not been investigated hitherto on chickens using an external parasite. MATERIALS AND METHODS