The in vitro and in vivo interaction of Schistosoma japonicum with salmonellae and other gram-negative bacteria was studied. In vitro, S. japonicum associated with salmonellae and other gram-negative bacteria, and more male than female schistosomes associated with the bacteria. By using the various strains and mutants of salmonellae, we showed that Salmonella typhimurium had a higher degree of association than did Salmonella enteritidis and that the piliated strains of S. typhimurium associated much more frequently than did the nonpiliated strains. However, in vivo studies demonstrated more frequent association of salmonellae with female than with male schistosomes and that the piliated and nonpiliated strains of salmonella did not differ in their association with the worms.