Seeded breadfruit (Artocarpus mariannensis) was historically a dominant tree in native forests on Guam and Rota in the Mariana Islands. Censuses conducted during 1989–1999 showed a large decline in the population of this species in northernmost Guam, with the number of trees at one study area decreasing from 549 to 190 trees, or 65.4%. Mean annual decline rates were far higher from 1989 to 1996 (9.2% per year) than from 1996 to 1999 (2.6% per year). Size structure of the population was strongly skewed toward larger trees, with 83.3% of measured individuals having trunk diameters ranging from 31 to 70 cm. Virtually no seedlings or saplings were present. Experiments at this site revealed high rates of fallen seed and fruit consumption and browsing on seedlings by introduced Philippine deer (Cervus mariannus) and feral pigs (Sus scrofa). In contrast, breadfruit populations elsewhere on Guam and Rota exhibited much less mortality. One population in an area without deer and pigs displayed considerable regeneration and a size structure composed mainly of younger plants. The decline of A. mariannensis in northern Guam appeared to be caused primarily by a combination of high mortality associated with an unusually severe typhoon season in 1992 and a nearly complete lack of recruitment due to excessive seed predation and herbivory by deer and pigs. Ungulate control is strongly urged to restore populations of A. mariannensis and other native plants, and to prevent further alteration of Guam's forests.