The intestinal pathogens Campylobacter jejuni and Campylobacter coli are the most common causes of foodborne bacterial gastroenteritis in humans. In Japan and globally, more than 90% of Campylobacter species isolated from patients with Campylobacter food poisoning are C. jejuni, whereas C. coli accounts for only a small percentage. This difference in isolation rates is considered to be due to differences in the ability of C. jejuni and C. coli to proliferate within the host. However, only a few studies have compared the growth of these two pathogens. To investigate the effect of C. jejuni on the proliferative ability of C. coli and vice versa, co-culture experiments were conducted. Similar strains were selected on the basis of their amino acid requirements for comparative purposes. C. jejuni grew on amino acid-rich media, indicating that its growth was not affected by the presence of C. coli. By contrast, the growth of C. coli was inhibited by C. jejuni. This suggests that the higher detection rate of C. jejuni may be due to its superior growth capacity rather than its initial abundance. Further research on C. coli is required to better understand its role and behavior in the host and in different environments.