Studies of dietary effects on heterosis for body weight gain (BWG) and feed conversion ratio (FCR) are few. In the present work, heterosis, heterosis due to sex chromosome, and maternal and line effects for BWG and FCR were compared in reciprocal crosses of White Plymouth Rock (WPR) and Single Comb White Leghorn (SCWL). These birds were reared on either 18 or 20% crude protein (CP) diets from hatch to 8 weeks. The WPR birds exhibited larger body weights and consumed more feed than the SCWL birds. Also WPR birds on 20% CP diet were larger than those on 18% CP diet. Although FCR of SCWL birds on 18% CP diet was lower than those on 20% CP diet at 1 and 3-6 weeks, FCR was lower in WPR than SCWL birds. Heterosis was higher at 18% CP diets and the average maternal and line effects for BWG were higher (P<0.05) for WPR than SCWL and at 18 than 20% CP diets. However, maternal effects for FCR of WPR progeny were higher at 20 than 18% CP diets. Average line effects were also higher for WPR than SCWL males and significant at 20% CP diets for both lines. Heterosis due to sex chromosome was significant (P<0.05) and positive for BWG but negative for FCR. This study indicates that SCWL males and WPR females exhibited higher combining ability at 18 than 20% CP diets. The significant heterosis for BWG and FCR due to sex chromosomes and maternal effects suggest that these two traits are also affected by genes on the sex chromosome.