Experimental populations of the redbacked vole, Clethrionomys rufocanus bedfordiae (Thomas), were introduced into field enclosures and then subsequently checked for home range overlap and reproductive condition. The times to parturition of females that did not have exclusive home ranges were significantly delayed compared with those of females whose ranges did not overlap (Table 4). The delays in parturition were not significantly related to the age of the females or to the number and ages of males that overlapped with a female. Copulatory plugs observed in the females that did not have exclusive ranges indicated that they often failed to become pregnant, regardless of having copulated. After the acquisition of exclusive ranges, these same females came into estrus and became pregnant (Figs. 2, 3).