Small mammal population fluctuations and aspects of demography were studied over more than 4 years on two 0.81 ha live-trapping grids in a precordilleran primary growth temperate rainforest in southern Chile. Seven species of sigmodontine rodents plus a putative hybrid, and two marsupials were trapped; the majority of recaptures were of four sigmodontine species: Akodon olivaceus, A. longipilis, A. sanborni, and Oryzomys longicaudatus. Maximum numbers occurred in January-July (late summer to winter months) and minimum in August-December (late winter to early summer months). Annual A. olivaceus numbers fluctuated strongly whereas A. longipilis and A. sanborni remained relatively stable. Numbers of O. longicaudatus were sporadic and irruptive. Reproduction was predominantly seasonal, in spring to fall months (September-April) for all species; most recruitment of young occurred in January-May for Akodon, but was apparently unrelated to reproduction in O. longicaudatus. Akodon had high 30-day survival rates and some individuals lived over two years; survival rates were generally low for O. longicaudatus. Additional live-trapping on peripheral lines suggested that movements of A. olivaceus during a period of population decline were predominantly unidirectional, but bidirectional for A. longipilis when their numbers were relatively stable. Comparisons with precordilleran Argentine forest sharing principal small mammal and dominant tree species indicates that both assemblages are similar in species diversity, densities, and in the chronology of reproduction and population change, but Chilean forests are dissimilar in being dominated by omnivorous Akodon, and O. longicaudatus, Historically, immigration from nearby forest areas may have resulted in greater homogeneity of small mammal assemblages in Chilean rainforests than in Argentine forests where immigrations from adjacent forests have been more restricted. Compositional differences in the faunas of the two forest regions appear to have been present over at least the last 10,000 yr.