The genome sequence of the food-borne pathogen *Campylobacter jejuni* reveals hypervariable sequences. The genome of *C. jejuni* NCTC11168 is 1,641,481 base pairs long and contains 1,654 predicted proteins. The genome is unusual in that it has very few repetitive sequences and no insertion sequences or phage-associated sequences. One of the most striking features is the presence of hypervariable sequences, which are short homopolymeric runs of nucleotides commonly found in genes involved in surface structure biosynthesis. These sequences may be important for the survival strategy of *C. jejuni*. *C. jejuni* is a microaerophilic, Gram-negative, flagellate, spiral bacterium that is the leading cause of bacterial food-borne diarrhoeal disease worldwide. It is also the most frequent cause of Guillain–Barré syndrome. The genome of *C. jejuni* is dense, with 94.3% of the genome coding for proteins. The genome has a high G+C content and is unusual in that it has few operons and clusters of genes. The genome also shows a high degree of variation in homopolymeric tracts, which may be associated with phase variation and may be produced by slipped-strand mispairing during replication. The genome of *C. jejuni* contains genes involved in the biosynthesis of lipooligosaccharides (LOS), extracellular polysaccharides (EP), and flagellar modification. These genes are clustered and are associated with hypervariable sequences. The genome also contains genes involved in iron acquisition, which are organized in operons under the control of the Fur protein. The genome has a unique set of regulatory genes, including members of the two-component regulator family. The genome also contains genes involved in the synthesis of sialic acid, which may be important for evasion of host immunity. The genome of *C. jejuni* is compared to that of *Helicobacter pylori*, which is closely related. The two organisms share many biological properties and are classified within the Campylobacter genus. However, they have distinct regulatory systems and are adapted to different ecological niches. The genome of *C. jejuni* is unique in that it contains a large number of biosynthetic genes not present in *H. pylori*, such as those involved in the synthesis of purines, thiamine, and many amino acids. The genome also contains genes involved in the synthesis of surface structures, which are associated with hypervariable sequences. The genome of *C. jejuni* provides the resources for a complete and detailed analysis of the pathogenic potential of this enigmatic pathogen. New insights into the biology of *C. jejuni* include the identification of hypervariable sequences, theThe genome sequence of the food-borne pathogen *Campylobacter jejuni* reveals hypervariable sequences. The genome of *C. jejuni* NCTC11168 is 1,641,481 base pairs long and contains 1,654 predicted proteins. The genome is unusual in that it has very few repetitive sequences and no insertion sequences or phage-associated sequences. One of the most striking features is the presence of hypervariable sequences, which are short homopolymeric runs of nucleotides commonly found in genes involved in surface structure biosynthesis. These sequences may be important for the survival strategy of *C. jejuni*. *C. jejuni* is a microaerophilic, Gram-negative, flagellate, spiral bacterium that is the leading cause of bacterial food-borne diarrhoeal disease worldwide. It is also the most frequent cause of Guillain–Barré syndrome. The genome of *C. jejuni* is dense, with 94.3% of the genome coding for proteins. The genome has a high G+C content and is unusual in that it has few operons and clusters of genes. The genome also shows a high degree of variation in homopolymeric tracts, which may be associated with phase variation and may be produced by slipped-strand mispairing during replication. The genome of *C. jejuni* contains genes involved in the biosynthesis of lipooligosaccharides (LOS), extracellular polysaccharides (EP), and flagellar modification. These genes are clustered and are associated with hypervariable sequences. The genome also contains genes involved in iron acquisition, which are organized in operons under the control of the Fur protein. The genome has a unique set of regulatory genes, including members of the two-component regulator family. The genome also contains genes involved in the synthesis of sialic acid, which may be important for evasion of host immunity. The genome of *C. jejuni* is compared to that of *Helicobacter pylori*, which is closely related. The two organisms share many biological properties and are classified within the Campylobacter genus. However, they have distinct regulatory systems and are adapted to different ecological niches. The genome of *C. jejuni* is unique in that it contains a large number of biosynthetic genes not present in *H. pylori*, such as those involved in the synthesis of purines, thiamine, and many amino acids. The genome also contains genes involved in the synthesis of surface structures, which are associated with hypervariable sequences. The genome of *C. jejuni* provides the resources for a complete and detailed analysis of the pathogenic potential of this enigmatic pathogen. New insights into the biology of *C. jejuni* include the identification of hypervariable sequences, the