It has previously been shown that Salmonella paratyphi A associates with adult Schistosoma mansoni worms in animal models and in patients with schistosomiasis, resulting in Salmonella infections that are difficult to manage unless the helminthic parasite is eliminated. In the present study, characteristics of this association were explored. In vitro, fresh serum, but not heat-inactivated (56 degree C, 30 minutes) serum, from normal hamsters (NH) and schistosome-infected hamsters (SIH) equally inhibited bacterial growth. When either group of animals was given S. paratyphi A intracardially, there was a decline in recoverable bacteria in the blood for 3 days, but after the third day, the number of bacteria increased in SIH whereas NH blood became sterile. Co-cultivation of S. paratyphi A in Earle's medium with live worms yielded more bacterial growth than occurred in the medium without worms, suggesting that nutritional factors are involved in the relationship. An intimate association in vivo of bacteria and trematodes was confirmed by culturing Salmonella from both worm surfaces and homogenized worms, with more bacteria being recovered from female worms. Although the bacterial infection induced a significant hepatic shift of adult worms in SIH, oogram analyses were normal and no lethal effect of bacteria on worms was noted.