||Crypsis is the color resemblance of an animal to its background such that predators have difficulty in distinguishing it. Although a great deal is known about crypsis in free-ranging insects, crypsis in ectoparasites is still poorly understood. Among ectoparasites, feather lice provide a unique chance to test whether ectoparasites are cryptically colored. Most feather lice are host specific and the major host defense against them is preening, a presumably visual process. Therefore, crypsis might be selectively advantageous for feather lice to escape from host defense. In this thesis, I use a comparative approach and an experimental approach to test for crypsis in feather lice. I investigate the correlation between louse color and host color. The data show that the color of congeneric lice is correlated with host colors while colors of head lice, which a bird cannot preen, are not. This suggests that feather lice are cryptically colored. I conducted experiments to test for crypsis using different color morphs of Rock Pigeons and their lice. I did not find evidence of crypsis from my experiments. Crypsis may simply not be important in Rock Pigeon lice. I show that feather lice do not reflect much ultraviolet using the reflectance spectra of pigeon lice. This thesis provides the first rigorous test of crypsis in ectoparasites.