The effects of morning (0900) vs. evening (2000) and natural (< 15 h natural light d-1) vs. extended (65 lux for 16 to 19 h d-1) photoperiod on vaginal temperature, growth rate and feed efficiency were examined in 48 crossbred beef heifers during the backgrounding and finishing periods. The backgrounding and finishing periods were each subdivided into two 28- intervals to relate ambient and vaginal temperature variables to feedlot performance variables. Time of feeding influenced performance and vaginal temperature patterns during the backgrounding period. Although evening-fed heifers spent more hours at vaginal temperatures exceeding daily mean vaginal temperature during the coldest part of the day (feeding time; P ≤ 0.03), morning-fed heifers grew faster and more efficiently than evening-fed heifers during the coldest interval of the backgrounding period (feeding time interval; P ≤ 0.01). Heifers exposed to an extended photoperiod had higher vaginal temperature, particularly in the late morning (0800 to 1100) than heifers receiving natural photoperiod during the finishing period. Results of this study indicated that the benefit of evening feeding on animal performance was more pronounced in cool than in cold weather. Key words: Beef cattle, thermoregulation, evening feeding