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 bat was collected in western Thailand in 1973 by Kitti Thonglongya, who died in February 1974 before publishing; his British colleague John Edwards Hill described it later that year and named the species in his honour. It is the sole member of the family Craseonycteridae.
The rival claimant is Suncus etruscus, the Etruscan shrew, which is longer in the body, at roughly 35 to 48 millimetres, but usually 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 tiny, spanning a limited area of western Thailand and neighbouring Myanmar, which is why disturbance at a handful of caves carries such weight for the species.