A Corynorhinus townsendii maternity roost located in an abandoned ranch house in central California was monitored for 25 y. Prior to the discovery of the bats in 1987, the house was broken into regularly and disturbance levels were quite high. Upon discovery of the roost, the house was fortified and vandalism was greatly reduced. The number of females and the number of volant young greatly increased during our study and was directly correlated with the decline in vandalism. Bats emerged from the house 43.6 (± 10.9 SD) min after local sunset. Bats emerged later in the evening during spring and fall, when it was warmer, and when it was windier. We also evaluated duration of emergence (47.11 [45.0–49.7] min), and seasonal patterns of re-entry into the roost. Several factors suggested that potential predation, most likely by owls, influenced both the timing and duration of evening emergences.