Bacteriophages infecting the food-borne human pathogen Campylobacter jejuni could potentially be exploited to reduce bacterial counts in poultry prior to slaughter. This bacterium colonizes the intestinal tract of poultry in high numbers, and contaminated poultry meat is regarded as the major source of human campylobacteriosis. In this study, we used phage F336 belonging to the Myoviridae family to select a C. jejuni NCTC11168 phage-resistant strain, called 11168R, with the aim of investigating the mechanisms of phage resistance. We found that phage F336 has reduced adsorption to 11168R, thus indicating that the receptor is altered. While proteinase K-treated C. jejuni cells did not affect adsorption, periodate treatment resulted in reduced adsorption, suggesting that the phage binds to a carbohydrate moiety. Using high-resolution magic angle spinning nuclear magnetic resonance (NMR) spectroscopy, we found that 11168R lacks an O-methyl phosphoramidate (MeOPN) moiety attached to the GalfNAc on the capsular polysaccharide (CPS), which was further confirmed by mass spectroscopy. Sequence analysis of 11168R showed that the potentially hypervariable gene cj1421, which encodes the GalfNAc MeOPN transferase, contains a tract of 10 Gs, resulting in a nonfunctional gene product. However, when 11168R reverted back to phage sensitive, cj1421 contained 9 Gs, and the GalfNAc MeOPN was regained in this strain. In summary, we have identified the phase-variable MeOPN moiety, a common component of the diverse capsular polysaccharides of C. jejuni, as a novel receptor of phages infecting this bacterium.