Conversion of Campylobacter upsaliensis to the coccoid form during aerobic incubation at 37 degrees C was not prevented by treatment with chloramphenicol and was accompanied by severe decreases in isocitrate dehydrogenase activity and oxygen uptake. Although the coccoid forms fluoresced orange-red by acridine orange staining, agarose gel electrophoresis indicated an extensive degradation of the ribosomal RNA. This suggests that acridine orange staining may not be a good indicator of viability and that the coccoid form of C. upsaliensis at 37 degrees C is degenerative rather than part of the life cycle.