Visits were made about once a week for 2 yrs. to a roost of bats, Pipistrellus pipistrellus, in a country church near Oxford. The largest count of bats emerging at dusk was 346. The threshold temp. for emergence was about 40[degree]F with light intensity at 1 foot-candle or less. Emergence came near sunset in Aug. and progressively later towards either end of the season, Feb. and Nov. The largest numbers were seen from May to Aug. Gusty winds and a heat wave caused reduced numbers to appear; the condition of the sky was unimportant.