Forty-eight Duroc X Hampshire crossbred pigs naturally infected with Strongyloides ransomi and Ascaris suum were used in a 2 X 2 X 2 X 2 experiment to study the effects of superimposed S. ransomi infection (experimentally exposed at an average weight and age of 18 kg and 66 days to 0 or 2,000,000 infective larvae per pig), sex (gilt and barrow), level of protein (high and low), and level of vitamin (high and low), on performance of growing-finishing swine. Because of extreme variability within treatments, postmortem recoveries of adult S. ransomi, when pigs averaged 92.6 kg, were not different (P > 0.10) even though exposed pigs averaged 424 and control pigs only 19. Sex and dietary protein and vitamin levels did not affect (P > 0.10) average daily gains (ADG) of pigs. The overall mean ADG of gilts and barrows were similar in magnitude; however, the effect of exposure on ADG of barrows was much different from that of gilts (P < 0.01). Exposed and control gilts had ADG of 0.75 and 0.73 kg, respectively; ADG of exposed and control barrows were 0.70 and 0.82 kg, respectively. The failure of S. ransomi infection, and levels of dietary protein and vitamins to exert a pronounced overall effect on daily feed intake was, in part, due to the interaction of sex and level of parasitism (P < 0.01) and the interaction of vitamin supplementation and level of parasitism (P <0.10). Artificial infection of pigs with S. ransomi had an effect on A. suum with an apparent parasite-sex and host-sex interaction. Nematode parasites in the digestive tract of animals must, to survive, take nourishment either from the tissues or ingesta of the host. In either case, the parasites should have a deleterious effect on the host performance, either directly by reduced weight gains, or indirectly by a reduced efficiency in converting feed into weight gain, or both. Harmful effects of parasitisms are exacerbated by low levels or deficiencies in protein, vitamins, or minerals in chickens (as reviewed by Chubb and Wakelin, 1963), in sheep (Whitlock, 1949; Gibson, 1963), in cattle (Vegors et al., 1955, 1956), and in pigs (Andrews and Jones, 1948; Kelley et al., 1958). Sex differences in susceptibility to nematode infections exist in various species of animals. However, the degree of susceptibility of a given sex appears to depend on the particular animal and parasite species involved. Both gonadal hormones and life cycle of the parasite may be involved in determining the success of a parasite infection (Dobson, 1964). Gastrointestinal helminths, when present in Received for publication 14 February 1969. Journal Series Paper No. 459 of the University of Georgia, College of Agriculture Experiment Stations, Coastal Plain Station, Tifton. * Animal Disease and Parasite Research Division, ARS, USDA, Tifton, Georgia 31794. t Animal Science Department, Georgia Coastal Plain Experiment Station, Tifton, Georgia 31794. large numbers, have repeatedly been shown to have the potential of reducing the efficiency and rate of gain of various domestic animals and of producing disease syndromes. However, there is no information on effects of parasitism on carcass traits of swine. This paper reports the results of an experiment designed to determine the effects of a superimposed Strongyloides ransomi infection on gilt and barrow crossbred pigs naturally infected with S. ransormi and Ascaris suum, fed diets with either of two levels of protein, each fortified with two levels of vitamins. Average daily gain, efficiency of feed conversion, and carcass quality were the principal measures used to evaluate the effects of the variables. MATERIALS AND METHODS