Beginning 5 weeks after being experimentally infested with known numbers of northern fowl mites, Ornithonyssus sylviarum (Canestrini and Fanzago), White Leghorn pullets caged alone supported a significantly higher mite population than did pullets housed two and three per cage. During the following 6 weeks, mite populations remained high on the birds caged singly whereas mite populations drastically declined on the birds housed two or three per cage. Eleven weeks after the experiment was initiated, all hens caged alone were infested with northern fowl mites, whereas 22% of the birds housed two per cage and 43% of the birds housed three per cage were free of mites. These data support the hypothesis that social stress in birds influences the development of northern fowl mites (more stress, higher resistance to mites).