Adult Craseonycteris thonglongyai measure roughly 29 to 33 millimetres from head to body end, with a wingspan near 15 centimetres and a mass of about 1.7 to 2 grams. There is no external tail. The species was described in 1974 by Kitti Thonglongya, who died before the paper was published, and it is the sole member of the family Craseonycteridae.
The rival claimant is Suncus etruscus, the Etruscan shrew, which is heavier in the body but often lighter overall, at around 1.8 grams. Which animal is smallest therefore depends on whether length or mass is the measure, and neither claim is wrong.
The bat roosts in limestone caves in colonies typically of a few dozen to a few hundred, and forages in short bouts at dusk and dawn, taking small insects near bamboo and teak. Its range is genuinely tiny, spanning a limited area of western Thailand and neighbouring Myanmar, which is why disturbance at a handful of caves carries so much weight for the species.