The aim of this study was to evaluate the effect of supplementary light on the performance of bulls in a feedlot. Feed intake (FI), average daily gain (ADG), back fat thickness (BFT), eye muscle area (EMA), P8 (fat layer on the rump), feed conversion ratio (FCR) and body development were measure in bulls exposed to different levels of light supplementation. Thirty young Bonsmara bulls from the same farm were randomly divided into three homogeneous groups of 10 animals each and subjected to one of three different levels of light treatment (16h, 24h and normal photoperiod). The additional light provided an average light intensity of 124 lux measured at eye level. The animals were housed in open pens and fed for 84 days ad libitum on a diet containing 11 MJ ME/kg DM and 14 g CP/kg. Ultrasound scanning was done using a PIE Medical Falco 100 scanner to measure subcutaneous fat depth between the 12th and 13th rib (BFT), longissimus dorsi (EMA) and P8 on days 1, 22, 51, 62 and 84 of the trial. Body measurements (body weight, body length and heart girth) were taken on the same days. The FI, ADG and FCR were calculated at the end of the trial. The results of this study demonstrated that light supplementation (24h light) significantly reduced FI and improved ADG and FCR, with no significant effect on the body measurements or subcutaneous fat accumulation (BFT and EMA). It was concluded that an extended photoperiod (EP = 24h) reduces FI but improves ADG and FCR of young beef bulls fed under intensive conditions.