Beef cattle birth weight data were analyzed by least-squares for estimation of environmental and heterotic effects. Sex, age of dam, year, season of birth, and breed or cross were all highly significant sources of variation in birth weight. Interactions of sex with breed or cross and of breed or cross with year were also highly significant. The interaction of breed or cross with age of dam approached significance. Average birth weight was 70.57 lb, Weights of pure Hereford and Brahman calves were 0.13 lb above and 8.25 lb below average, respectively. Firstcross calves from Hereford dams were 12.75 lb above the mean while those from Brahman dams were 6.46 lb below average. Weights of the various other types of crossbred calves varied considerably, with genotype of dam exerting a large influence. Hereford dams had heavier calves than did Brahman or firstcross dams. Genotype of sire exerted only moderate effects on birth weight. Heterosis was 10.8% for firstcross calves, 5.5% for backcross calves from firstcross cows, 8.2% for backcross calves from purebred cows, and 2.0% for F2 calves.