Cross-sectional, longitudinal, and case-control studies were conducted to describe the epidemiology of Campylobacter in chickens, swine, dairy cows, farm workers, nonfarm residents, and children with diarrhea. Samples were collected in Chiang Mai and Lamphung provinces of northern Thailand from 2000 through 2003. A total of 2,360 samples were processed. Results from the cross-sectional study indicated that the prevalences of Campylobacter in chickens at the farm, slaughterhouse, and market were 64, 38, and 47%, respectively. In swine, the prevalences at the farm, slaughterhouse, and market were 73, 69, and 23%, respectively. Campylobacter prevalence was 14% in dairy cows and 5% in raw milk. The prevalence of Campylobacter on farms was lower in environmental samples than in samples collected from live animals. No Campylobacter isolates were obtained from healthy nonfarm residents, but isolates were obtained from 5 and 18% of farm workers and children with diarrhea, respectively. The prevalence of Campylobacter in pigs in the longitudinal study was 61% at the farm, 46% at the slaughterhouse, and 33% at the market. The majority of Campylobacter isolates from chickens (52%), swine (98%), and farm workers (66%) were Campylobacter coli, whereas the majority of isolates from dairy cows (63%) and children with diarrhea (62%) were Campylobacter jejuni. Most Campylobacter isolates from diarrheal children had single-strand conformation polymorphism profiles similar to those of isolates from chickens. None of the risk factors for infection in children with diarrhea were significantly associated with the isolation of Campylobacter.