Myomorph rodents play important roles in trophic systems and can have rapid population-level responses to food pulses, such as mast. The purpose of our study was to measure such responses and record potential interactions among the rodent species in a northern hardwood forest. We used mark—recapture methods to estimate abundances of 3 myomorphs commonly found in northern hardwood forests—Peromyscus spp. (deer mice, hereafter, Peromyscus), Napaeozapus insignis (Woodland Jumping Mouse), and Myodes gapperi (Red-backed Vole)—over 2 years (2006 and 2007). Seedfall was measured concurrently. The abundance of Peromyscus and Red-backed Voles substantially increased in response to the 2006 mast, which was the highest in 7 years of continuously recorded data at Hubbard Brook Experimental Forest. Adult-mean weights of all 3 species were higher during the spring following mast production than during the preceding spring. Following these responses to the mast and during the subsequent lean summer of 2007, Woodland Jumping Mice completely disappeared from the study area, mean Peromyscus weight dropped to its lowest level during the study, and reproductive activity of Peromyscus and Red-backed Voles substantially declined. Woodland Jumping Mice likely dispersed from the area in response to interference competition from a substantially increased Peromyscus population. These pulses in food, particularly the larger seeds of Fagus grandifolia (American Beech), and the consequent population responses corroborate the patterns found by others and may influence the food webs of these northern-hardwood forest communities beyond the trophic level of granivores.