The heritability of birth weight was estimated in 3,562 captive pigtailed macaques using 30 years of breeding and pedigree records. Based on a pedigree of over 12,000 animals, quantitative genetic analyses were performed using statistical variance decomposition methods. The model included additive genetic effects, cytoplasmic genetic effects, birth environment, shared maternal environment, and unmeasured environmental effects. The results demonstrated a strong (h2 = 0.51) heritable component of birth weight overall, and included significant additive genetic heritability (h2 = 0.23), and cytoplasmic heritability (h2 = 0.09). In addition, a significant effect of birth location and cage type was identified, explaining an additional 6% of birth weight variance. The use of a nonhuman primate model for studying the effects of genes on birth weight eliminated many of the problems associated with confounding variables in human studies, and allowed for the quantification of a heritable component of birth weight. Am. J. Primatol. 56:207–213, 2002. © 2002 Wiley-Liss, Inc.