We have hypothesized that stress-induced subclinical infection of turkeys with Listeria monocytogenes may be an overlooked source of processing plant contamination, and we have shown that concurrent Escherichia coli challenge can increase L. monocytogenes colonization. The objective of this study was to determine the effects of dexamethasone (Dex) immunosuppressive treatment and transport stress on the isolation of L. monocytogenes in an E. coli–L. monocytogenes challenge model. Thirteen-week-old male turkeys housed in floor pens were either nonchallenged (NCH) or challenged (CH) by environmental exposure to E. coli and L. monocytogenes Scott A, using both a coarse spray and feed inclusion. One group of both NCH and CH birds was not stressed (NCH-Con and CH-Con, respectively), a second group was treated with Dex during challenge (NCH-Dex and CH-Dex, respectively), and a third group was subjected to a 12-hour transport (Trans) stress protocol at 15 wk of age (NCH-Trans and CH-Trans, respectively). All birds were bled and necropsied the morning after transport. Dexamethasone treatment increased mortality and disease incidence. The CH-Con, CH-Trans, and CH-Dex birds, as well as the NCH-Dex birds, had lowered body weights compared to the NCH-Con. The relative liver and heart weights were increased, and the relative bursal weights were decreased by both NCH-Dex and CH-Dex treatments. The heterophil:lymphocyte ratio, a measure of the stress response, was increased by CH-Trans, NCH-Dex, and CH-Dex. Total erythrocyte counts and hematocrit were decreased by NCH-Trans, CH-Trans, NCH-Dex and CH-Dex. The challenge strain of E. coli was isolated from the organs of a significant number of CH-Dex birds using direct plating and occasionally from CH-Trans birds. Listeria monocytogenes was not isolated from significant numbers of birds using direct plating, but was isolated from the knee or hip synovial tissues of a significant percentage of CH-Dex birds using pre-enrichment cultural methods, and from CH-Trans and CH-Dex birds using real-time PCR detection. These data suggest that L. monocytogenes colonization of processing-age turkeys can be increased by stress, and the organism may be harbored within inapparent infections of turkey synovial tissue.