Two host-adapted strains of Nippostrongylus brasiliensis (Travassos, 1914) that had been isolated from the same stock infection of laboratory rats, and thereafter maintained exclusively in either rats or Syrian hamsters for over 150 nematode generations, were used to study the immediate and longterm influence of host species on the phenotype of the nematode. Thirteen nematode characters were measured at 8 necropsy times from the 2nd to 13th days of infection. Observations also were obtained of the percentage development, growth, egg production, and length of survival in rats and hamsters. When raised in hamsters, both strains had significantly longer spicules than when raised in rats. In contrast, the hamster-adapted strain was significantly shorter in body length with significantly shorter spicules than the rat strain regardless of host. The size differences between the strains were found to be the result of different patterns of growth. Because: (1) hamster-adapted strain females showed a reduction in the number of eggs in the uterus earlier in hamsters than in rats; and (2) rat strain nematodes persisted longer in the intestines of rats than the hamster-adapted strain did in either rats or hamsters-it is suggested that the hamster develops an immune response to the nematode more quickly than does the rat. This factor may have resulted in selection for nematodes that complete growth more rapidly and have a shorter life-span. The classification of parasitic nematodes, like that of most animals, is based largely on an analysis of adult structure. The study of variations of the phenotype is, therefore, basic to determining whether the classification is sound. The challenge lies in determining whether phenotypic variations reflect differences in the genotype or if they are due to differences in the environment. One aspect of the environment that can significantly affect the phenotype of parasitic nematodes is host species (Scott, 1929; Herlich, 1958; Neafie and Haley, 1962; Cross, 1965; Knight, 1968). Host-adapted strains have been reported in many nematode species; for a review see Chitwood (1957) and Haley (1962). Intraspecific strains have been developed in the laboratory by serial animal passage (Taylor, 1928; Scott, 1930; Foster and Cort, 1937; Haley, 1958, 1966a, b; Wood and HanReceived for publication November 3 1970. * From a dissertation submitted to the Graduate School of the University of Maryland, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This work was supported in part by a National Science Foundation Traineeship and by research grant A103388 from the NIAID, NIH, USPHS. t Present address: National Animal Parasite Laboratory, Veterinary Sciences Research Division, ARS, USDA, Beltsville, Maryland 20705. en, 1960; Bezubik, 1962; Solomon and Haley, 1966). Haley (1962) suggested the use of laboratory-developed nematode strains as models of intraspecific populations to study the interaction of environment and genotype. He developed several host-adapted strains of Nippostrongylus brasiliensis (Travassos, 1914), an intestinal nematode of the Norway rat, and reported differences in infectivity, development, sex ratio, and egg production, in hosts of different species, age, and sex (Haley, 1966a, b). However, no study has previously been made of the morphology of these strains. Reported herein are the results of a study to determine any phenotypic differences that might have developed between a hamsteradapted strain and a laboratory rat strain of N. brasiliensis that had been maintained separately in their respective host species for more than 150 nematode generations. Each of the two nematode strains was raised in both rats and hamsters. Comparing samples of the two strains from the same host had the effect of holding the environment (the host) constant, allowing genetic differences between the strains to be identified. Comparing samples of the same strain from different host species permitted a determination of immediate host effects on the nematode strains.