The gastrointestinal bacterium Klebsiella (née Enterobacter) aerogenes expresses an endogenously generated, temperature-compensated circadian rhythm in swarming motility. We hypothesized that this rhythm may be synchronized/entrained in vivo by body temperature (T). To determine entrainment, cultures expressing bioluminescence were exposed to temperature cycles of 1°C (35°C-36°C) or 3°C (34°C-37°C) in amplitude at periods (T-cycles) of T = 22, T = 24, or T = 28 h. Bacteria entrained to all T-cycles at both amplitudes and with stable phase relationships. A high-amplitude phase response curve (PRC) in response to 1-h pulses of 3°C temperature spike (34°C-37°C) at different circadian phases was constructed, revealing a Type-0 phase resetting paradigm. Furthermore, real-time bioluminescence imaging revealed a spatiotemporal pattern to the circadian rhythm. These data are consistent with the hypothesis that the K. aerogenes circadian clock entrains to its host via detection of and phase shifting to the daily pattern of T.