We tested the hypothesis that exposure of Northern Bobwhites (Colinus virginianus) to acute temperature stressors impairs their immune-system function and resistance to disease. Birds were randomly assigned to either a thermoneutral (constant 21°C), cold-stress (cycled from 3.6 to -20°C over 24 h), or heat-stress (cycled from 30.8 to 39.0°C over 24 h) treatment for four consecutive days. Immunocompetence of each bird was evaluated on day 5 using a panel of assays, including a measure of resistance to a Pasteurella multocida Type 3 challenge. Neither thermal stressor had an influence on spleen mass or measures of cell-mediated and humoral immunity. Disease resistance of Northern Bobwhites to P. multocida was not influenced by heat stress but increased following cold stress. Cold stress may have increased the activity of phagocytic leukocytes, which are important in resistance of birds to bacterial pathogens. A concomitant decrease in lymphocyte numbers suggested that resistance to vital pathogens was compromised during cold stress.