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In November 1960, bat colonies were found in the roof of the research station, and an ecological study began of the largest, a Plecotus colony. That colony was at first thought to consist solely of P. auritus but, in 1964, the presence of P. austriacus was confirmed, and several of the latter had been ringed early in 1961. Because of this, it has been necessary to present, here, the combined results for the two species. Population studies revealed that the colony size was constant and consisted of 36 bats of which 23 were females. Overall 38% were males but, of the juveniles, 46% were males. The survival rate for the colony as a whole was 0.750, but the mean life expectation for females was 4.2 years and for males 3.7 years. The life-span was thought to be in excess of 16 years. Initial capture rate of 0.715 remained constant throughout the five-year period, while the recapture rate fell by a factor of 0.30. This was attributed to sampling disturbance and not due to ringing. Mating was found to occur equally in spring and autumn. Males became mature in their first year and females in their second. The mean spring and autumn weights for mature males were generally 1.5g less than the corresponding mature female weights, and juvenilesare approximately 1.0 g less than these. Climatic effects on the bats were demonstrated with particular reference to wind velocity. This work is beingcontinued.