We examined the relationship between food patch size and feeding party size with comparative data from two populations of muriquis (Brachyteles arachnoides) in the 37,797-ha forest at the Parque Estadual de Carlos Botelho (PECB), São Paulo, and the 800-ha forest at the Estação Biológica de Caratinga (EBC), Minas Gerais. Precipitation was more abundant and less seasonal at PECB than EBC, and the density of large trees (>25.0 cm) was higher at PECB (206 ha-1) than at EBC (132 ha-1). At both sites, the size of feeding parties is positively related to the size of food patches. As predicted, food patches at PECB are significantly larger than those at EBC for both fruit and leaf sources. Contrary to expectations, feeding parties were larger at EBC than PECB. The higher population density of muriquis and sympatric primates at EBC may make large associations more advantageous to these muriquis than to muriquis living at lower population densities in PECB.