Thanasimus formicarius (L.), a main predator of bark beetles in Eurasia, was studied at constant temperatures of 15, 21, and 25 °C. Morphological data and duration of development are indicated for the egg (2.4 by 0.8 mm; 28–7 days at temperatures from 15 to 25 °C), three larval instars (9–14 days depending on instar and temperatures of 21 and 25 °C), pupa (15 and 33.5 days over the range of temperatures), and adult. Information is also given on head capsule size, sex ratio, fecundity, and length of adult life.