During 1965 and 1966, 957 bats were banded in a Tadarida brasiliensis colony in southeastern Louisiana. Of these, about one third were recovered in the colony one or more times. Banded individuals were also recovered or observed in several cities and towns up to 85 miles from the home roost. Of the two major subcolonies, one reached population peaks in midsummer and late autumn, the other in early autumn. The bats were known to be present in all months except January and February, during which time they had probably retreated to inaccessible wall spaces. In the nursery colony males, females and young were segregated, but the pattern of segregation broke down when the young began to fly in July. Apparently, some mothers flew with their young from one building to another, or the young made this flight as soon as they were able to fly. Bats refrigerated at 13.5 C entered torpor, but had no difficulty achieving normal body temperature when removed. Bats in the colony entered torpor only during periods of cool weather. Females outweighed males during most of the year, but both sexes gained weight during the autumn. Three bats banded in the colony were known to have lived at least 6 V/2 years, and one individual was at least 8 years of age when killed. INTRODUCTION The scant literature dealing with Tadarida in the southeastern United States was reviewed by Barbour and W. H. Davis (1969). W. B. Davis (1966) considered southeastern Tadarida to represent a distinct species, T. cynocephata. Although Barbour and Davis included the southeastern population in the wide-ranging T. brasiliensis, they suggested that the southeastern and southwestern populations may be reproductively isolated, but reached no taxonomic conclusions. No studies of colony organization and movements of T. b. cynocephala have been published. By banding a larg-e number of bats and observing the colony throughout the year, I collected data which permitted me to describe the population dynamics, social organization and movements of a Tadarida colony, and to learn something of the reproduction, development and longevity of the southeastern subspecies. MATERIALS AND METHODS From early May 1965 to late November 1966 a colony of Tadarida brasiliensis in Clinton, La., was visited at frequent but irregular intervals, and 867 adult and 90 juvenal bats were banded. Body and air temperatures were recorded with a YSI model 42SC Telethermometer and an anal probe. Weights were recorded using a standard laboratory balance calibrated in tenths of grams. The colony was divided among three old, presently nonresidential buildings, all wlthin a three-block radius. They will henceforth be