Abstract: Swarming behaviour of Leptothorax tuberum (Fabricius, 1775) in the field. - On July 28, 2002 swarming behaviour of Leptothorax tuberum was observed in a pasture at ca 1350 m a.s.l., at Monte Baldo in Italy. Small groups of 2 - 10 males assembled on top of rock piles ca. 1 - 3 m in diameter and little higher than the surrounding grass. Only males were seen between about 09.30 and 10.30 (Central European Time), with temperatures of ca 20°C and a light breeze, and alternating clouds and sunshine. Sometimes the males engaged in homosexual behaviour. The males tried to mount a female immediately after it was artificially put into a swarm. Squeezed females exposed in a swarm also elicited mounting behaviour of males. Male swarms were seen until ca 14.00, and alate as well as dealate females were found in the afternoon, crawling across rocks or trying to hide in rock crevices.