The morphological transformation from the spiral form to the coccoidal form Campylobacter jejuni and Campylobacter coli was studied under various conditions by such techniques as electron microscopy, SDS-polyacrylamide gel electrophoresis (SDS-PAGE), and chemical analyses. The conversion from the spiral form to the coccoidal form of Campylobacter in microaerophilic cultivation occurred in about 50% of the cells in 48 h and in about 90% of the cells in 72 h. At higher temperatures (37 C and 42 C) under aerophilic conditions, the spiral-form cells converted easily to the coccoidal form in phosphate-buffered saline. Electron-microscopic studies revealed that the envelopes of the coccoidal-form cells were soft in comparison with those of the spiral-form cells. The LPS and protein contents of the cells reached the highest levels after cultivation for 48 h under microaerophilic condition. The 33 K and 28 K polypeptide contents of 24-h and 48-h cultures were higher than those of 72-h and 96-h cultures.