A new bat, Karstala silva, gen. et sp. nov., is described from the early Miocene (early Hemingfordian) Thomas Farm local fauna, Gilchrist County, Florida. The new taxon is represented by an upper canine, posterior part of the dentary, isolated upper and lower molars, much of the humerus, and part of the radius. The myotodont configuration and steeply inclined lingual cingulid of m1-m2 separate Karstala from the Molossidae and most Vespertilionidae except Antrozous. Karstala differs from Antrozous in other molar details and from all molossids and North American vespertilionids in having a humerus with the combination of a rounded head, deep supraglenoid pit, long and high pectoral ridge, tiny olecranon fossa, and no notch between the medial side of the trochlea and spinous process. The new species is comparable in size to the larger known species of the Vespertilionidae and is the largest known bat in the Tertiary of North America.