Campylobacter jejuni, when left on blood agar for prolonged periods, was found to survive better in air than under micro-aerobic conditions. After a period of 2-3 days in air, all strains of C. jejuni examined grew freely in air on subculture, and could be further subcultured apparently indefinitely in air. This adaptation to aerobic metabolism was accompanied by a change in colony morphology and some changes in outer-membrane protein patterns, but no change in serotyping reactions. The ability to colonise mice was unaltered as was the helical morphology of growing cells. The important survival phase of C. jejuni, when outside the animal gut, involves not only a change to coccal morphology but also fundamental changes in the metabolism of the organism. These changes are likely to be relevant to techniques required for culturing C. jejuni from foods and environmental sources.