Control of Campylobacter in the food chain requires a better understanding of the behaviour of the bacteria in relevant environments. Campylobacter species are largely non-pathogenic in poultry, the body temperature of which is 42 A degrees C. However, the bacteria are highly pathogenic in humans whose body temperature is 37 A degrees C. The aim of this study was to examine if switching from commensal to pathogenic behaviour was related to temperature. We examined the growth, motility and invasion of T84 cells by three species of Campylobacter: C. jejuni 81116, C. jejuni M1, C. coli 1669, C. coli RM2228 and C. fetus fetus NC10842 grown at 37 and 42 A degrees C. Our results suggest that C. jejuni isolates grow similarly at both temperatures but some are more motile at 42 A degrees C and some are more invasive at 37 A degrees C, which may account for its rapid spread in poultry flocks and for infection in humans, respectively. C. coli, which are infrequent causes of Campylobacter infections in humans, is less able to grow and move at 37 A degrees C compared to 42 A degrees C but was significantly more invasive at the lower temperature. C. fetus fetus, which is infrequently found in poultry, is less able to grow and invade at 42 A degrees C.