We describe the resource availability and diet of western lowland gorillas (Gorilla gorilla gorilla) from a new study site in the Central African Republic and Republic of Congo based on 3 years of study. The results, based on 715 fecal samples and 617 days of feeding trails, were similar to those reported from three other sites, in spite of differences in herb and fruit availability. Staple foods (consumed year-round) included high-quality herbs (Haumania), swamp herbs (when present), and a minimal diversity of fruit. A variety of fruits (average of 3.5 species per day and 10 per month) were selectively consumed; gorillas ignored some common fruits and incorporated rare fruits to a degree higher than predicted based on availability. During periods of fruit abundance, fruit constituted most of the diet. When succulent fruits were unavailable, gorillas used low-quality herbs (i.e., low-protein), bark, and more fibrous fruits as fallback foods. Fibrous fruit species, such as Duboscia macrocarpa and Klainedoxa gabonensis, were particularly important to gorillas at Mondika and other sites as fallbacks. The densities of these two species are similar across sites for which data are available, in spite of major differences in forest structure, suggesting they may be key species in determining gorilla density. No sex difference in diet was detected. Such little variation in western lowland gorilla diet across sites and between sexes was unexpected and may partly reflect limitations of indirect sampling. Am. J. Primatol. 58:91–116, 2002. © 2002 Wiley-Liss, Inc.