There are an estimated 2 million cases of salmonellosis in the United States every year. Unlike the incidence of many infectious diseases, the incidence of salmonellosis in the United States and other developed countries has been rising steadily over the past 30 years, and the disease now accounts for 10 to 15% of all cases of acute gastroenteritis in the United States. The infecting organism is ingested and must traverse the intestinal epithelium to reach its preferred site for multiplication, the reticuloendothelial system. Despite several recent studies, the genetic basis of the invasion process is poorly understood. An emerging theme from these studies is that wild-type Salmonella organisms probably have several chromosomal loci that are required for the most efficient level of invasion. In this study, we have identified and characterized 13 TnphoA insertion mutants of Salmonella enteritidis CDC5 that exhibit altered invasion phenotypes. The mutants were identified by screening a bank of TnphoA insertions in S. enteritidis CDC5str for their invasion phenotype in three tissue culture cell lines (HEp-2, CHO, and MDCK). These 13 mutants were separated into six classes based on their invasive phenotypes in the tissue culture cell lines. Several mutants were defective for entry of some cell lines but not for others, while two mutants (SM6 and SM7) were defective for entry into all three tissue culture cell lines. This suggests that Salmonella spp. may express more than one invasion pathway. Southern analysis and chromosomal mapping indicated that as many as nine chromosomal loci may contribute to the invasion phenotype. It is becoming clear that the invasive phenotype of Salmonella spp. is multifactorial and more complex than that of some other invasive members of the family Enterobacteriaceae.