Technical Abstract: The objective of the following experiment was to examine the effect of light exposure on horizontal transmission of Salmonella typhimurium in weaned pigs. Twenty crossbred pigs (average BW = 15 kg) were housed in isolation rooms (10 pigs/room) and randomly assigned to one of two lighting regimes: Low (8 h light, 16 h dark) or High (16 h light, 8 h dark). Pigs were adjusted to their respective lighting treatments for six days and on the seventh day, two randomly selected pigs/room orally inoculated with 5 ml of tryptic soy broth containing 18 x 108 cfu Salmonella typhimurium/ml. Rectal swabs were collected from each pig daily over the next eight days for direct plating and plating following 24-h enrichment. On day nine, following inoculation of the seeder pigs, all pigs were euthanized and necropsied. Luminal contents were collected from the ileum, colon, cecum and rectum (quantification and qualification of inoculated strain) and tissue samples collected from the above gut segments as well as the tonsils, ileo-cecal lymph nodes, spleen and liver (qualification only). The number of rectal swabs positive for the inoculation strain of Salmonella was higher (P = 0.003) in the High lighting treatment (25 versus 5.4% positive) compared to the Low treatment when examined across sampling days. No differences (P > 0.10) were observed in the percentage of fecal swabs positive for Salmonella following enrichment. Serial dilutions of luminal contents were not statistically different with very few samples containing quantifiable amounts of Salmonella. However, following enrichment, the percentage of positive luminal content samples was higher (P < 0.05) in the colon and rectum and tended to be higher (P = 0.07) in the cecum, in pigs exposed to 16 h of light. The percentage of tissue samples from the ileum and colon that were Salmonella positive was also higher (P < 0.05) in the High lighting treatment compared to those receiving 8 hours light. No other differences were observed in tissue or luminal contents. Body weights were similar among treatments prior to initiation of lighting treatments and weight gains were not different (P > 0.10) among treatments. Results of this research indicate that lighting exposure may play a role in the horizontal transmission of Salmonella typhimurium among weaned pigs.