Wingspan: 9.5–10.5 in.
Length: 2.5–4 in.
Weight: 0.14–0.32 oz
Color: A small bat with dense, dull, woolly fur; upperparts brownish to sooty; fur of underparts with white tips and black bases, the general white appearance contrasting sharply with the upperparts.
Roosting behavior: Hibernates in northern climates but is active throughout the year in southern areas of distribution. Pregnant females form maternity colonies, usually in caves over water. Most females bear twins.