Captive flocks of red jungle fowl (Gallus gallus) experimentally infected with the intestinal nematode Ascaridia galli were used to test Hamilton and Zuk's (1982) hypothesis that parasites adversely affect male secondary sex characters and that females prefer unparasitized over parasitized males. Infected chicks grew more slowly than uninfected controls, with the effect particularly pronounced on comb length rather than tarsus length or body weight. At sexual maturity, infected roosters had duller combs and eyes, shorter combs and tail feathers, and paler hackle feathers than control roosters. In experimental mate choice tests, females preferred unparasitized over parasitized roosters by about 2: 1, and an analysis of covariance revealed that the hens were using the traits on which the two groups differed to make their mate choice decisions. Finally, in a test of an extension of the Hamilton and Zuk hypothesis, control and infected males were not distinguishable based on non sexually-selected characters such as bill size, suggesting that parasites have a disproportionately larger effect on ornamental traits.