Salmonellosis is one of the most important wasting diseases that leads to economic damage in the swine industry. Many risk factors have been reported to increase the spread of Salmonella infection; therefore, it is important to understand how to treat the risk factors of Salmonella to effectively prevent salmonellosis in commercial pig farms. To accomplish this, we conducted a study to determine if the seasons and porcine production stages affected the serological response to Salmonella in Korea. A total of 1,592 serum samples submitted to the School of Veterinary Medicine of Kangwon National University between Jan. 2001 and Dec. 2004 from commercial farms were tested by ELISA. The overall apparent seroprevalence of salmonellosis was 38.1% (95% CI, 38.0-38.2), while the prevalence of Salmonella according to seasons and production stages ranged from 17.9% to 62.8% for the former (24.6% in spring, 17.9% in summer, 38.5% in autumn, and 62.8% in winter) and from 16.1% to 68.3% for the latter (17.9% in suckling pigs, 16.1% in weaning pigs, 37.50% in growers, 41.9% in finishers, 48.0% in gilts, and 68.3% in sows). In this study of seroprevalence by production stage, most pigs were naturally infected by Salmonella during the weaning stage. Also, seroprevalences were found to have a seasonal pattern in which most pigs were infected in autumn to winter.