Helicobacter pylori strains RH 54 and NCTC 11637 were grown in brain-heart infusion broth up to 56 days, and the coccoid form was obtained during prolonged incubation. Two morphological types of coccoids were observed, one of which was electron-dense and had an intact cellular membrane and flagella, indicating that it was likely to be viable. The other coccoid form was sphaeroblast-like and weakly stained, showing features of degeneration. Catalase activity was positive for aged cultures even up to 160 days. Sodium dodecyl sulphate polyacrylamide gel electrophoresis showed that most of the protein bands appeared to be similar in both the spiral and coccoid forms. In addition, Lewis blood group antigens were detected in cultures of up to 8 weeks. Furthermore, two sets of primers for the vacA and cagA genes were used in polymerase chain reaction, and these two important genes remained conserved in both the spiral and coccoid forms. The present study shows that the coccoid form of H. pylori retained many important characteristics present in the spiral form despite the morphological conversion, and thus supports the notion that some of the coccoid forms of H. pylori are likely to be viable.