The Great Evening Bat (Ia io) is a bat in the vesper bat family (Vespertilionidae) and the only species in the genus Ia. It is common to Eastern and Southeastern Asia (China, India, Laos, Nepal, Thailand and Vietnam), mainly living in areas with limestone caves at altitudes of . Their roost sites have been found both near the cave entrances and up to within the cave systems.
The Great Evening Bat reaches a length of . It is colored brown on the top and grayish on the bottom.
Not much is known about its habits and behavior. The bat usually lives in small groups. Its food consists of insects, as with most vesper bats. The bat leaves its sleeping place already in the late afternoon for the search of food. During the winter months it may migrate to warmer regions.
The IUCN lists its conservation status as lower risk, near threatened. One of the threats to its survival is human influence by habitat destruction; many caves have been turned into attractions. They have also been disturbed by farmers collecting their excrement. Also the excessive use of insecticides poses a threat to the Great Evening Bats.