The adults of Micropterna spp. estivate in caves, but there are considerable differences in abundance and species composition among different caves. Marking-recapture experiments were carried out with M. nycterobia near and in the Eisenstein cave in Lower Austria. Individuals released in early summer in distances up to 3 km from the cave returned and were recaptured there at rates of only 1-5%, but more or less independent of the distance from place of release and of the individual date of emergence. Later in the summer released Micropterna no longer immigrated. It is supposed that the migration of the adults of this species is not exclusively directed to caves, and that high abundance in a given cave depends mainly on mass occurrence of the larval stages in a nearby running stream. By exposure to light-sensitive material it was demonstrated that the caddis flies rest in the cave only in the region of weak illumination. They avoid total darkness. Therefore, we have not excluded that the development of their gonads depends, as is usual in insects, on day length.