Light environments (determined by the factors of light source, wavelength, intensity, and photoperiod) in commercial poultry houses play important roles in growth, maintenance, and performance of chickens. In modern houses, producers have greater control over the light environment, particularly photoperiod. Manipulating photoperiod can be an effective way to alter behavior and optimize performance. The present studies were conducted to evaluate the effects of 3 different photoperiod regimens (20 h, 18 h, and SD/SU 12L:12D) on performance and behavior of Cobb 500 broilers. Based on the data, the SD/SU program resulted in slower growth rates compared to the 20 h and 18 h treatments to 3 wk of age; however broilers in all treatments finished with comparable 6 wk BW. Behaviorally, at 22 d, broilers under the SD/SU treatment (12L:12D at this age) were more active during the dark period than those under either the 20 h or 18 h treatments. However, the amount of time partitioned for each of the behaviors over a 24 h day was not altered. There were no statistically significant differences in gait scores at 6 wk or heterophil:lymphocyte (H:L) ratios at 38 d for the 3 photoperiod treatments.