Response of eight Azolla strains from six species to transient exposure to high-temperature stress (above 40 degrees C) was examined. Each Azolla strain showed differential tolerances to the stress. Based on growth and survival after the treatment, we concluded that the order of tolerance of six species to transient high-temperature stress was as follows: A. pinnata>A, microphylla, A. mexicana>A. caroliniana, A. filiculoides>A. rubra. Treatment with high-temperature stress also caused the rapid abscission of root and branches of Azolla. The temperature and time that caused rapid abscission depended on the species. In A. filiculoides and A. microphylla, the abscised branches were alive and proliferative when the temperature stress was short-term. (C) 1999 Elsevier Science B.V. All rights reserved.