Abstract: After floods in July 1997 in eastern Austria mynnecological observations at two rivers with different conditions of inundation were conducted. One day after the flood discharge the ant fauna on the bank of River Traisen (St. Pölten, Lower Austria) appeared nearly undamaged. In a 10 hours flooded area of 335 m2 nests of 10 ant species were observed. In the flooded forests of River Danube (Lob au , Vienna), individuals of Myrmica rubra, Leptothorax nylanderi slavonicus and Lasius juliginosus were observed outside their nests, floating on the water or attached to drifting material. Only Myrmica rubra formed floating aggregations (" Ameisentrauben") of workers and queens. This behaviour of Myrmica rubra is reported here for the first time. Similar floating aggregations are also known of Myrmica gallienii and are interpreted as an adaptation to periodic floods.