A prospective cohort study was performed to evaluate the prevalences and loads of Salmonella and Campylobacter spp. in farm and processing plant samples collected from 55 commercial broiler chicken flocks. Environmental samples were collected from broiler houseswithin 48 h before slaughter, and carcass rinses were performed on birds from the same flocks at 4 different stages of processing. Salmonella was detected in farm samples of 50 (90.9%) flocks and in processing samples of 52 (94.5%) flocks. Campylobacter was detected in farm samples of 35 (63.6%) flocks and in processing samples of 48 (87.3%) flocks. There was a significant positive relationship between environmental farm samples and processing plant carcass rinses with respect to both Salmonella and Campylobacter prevalences andloads. Campylobacter loads were significantly higher than Salmonella loads, and the correlations between samples collected from the same flocks were higher for Campylobacter thanthey were for Salmonella. Boot socks were the most sensitive sample type for detection ofSalmonella on the farm, whereas litter samples had the strongest association with Salmonella loads in pre- and postchill carcass rinses. Boot socks, drag swabs, and fecal samplesall had similar sensitivities for detecting Campylobacter on the farm, and all were more strongly associated with Campylobacter loads in carcass rinses than were litter samples. Farm samples explained a greater proportion of the variability in carcass rinse prevalences and loads for Campylobacter than they did for Salmonella. Salmonella and Campylobacterprevalences and loads both decreased significantly as birds progressed through the processing plant.