The waters from the radioactive thermal springs at the lower western slope of mount Graukogel arrive at the Austrian spa Bad Hofgastein via an eternite pipeline and are then distributed to various localities by means of (smaller) iron tubes. The main duct from the springs to the spa, as well as the local distribution system, were both found to contain sediments. They are formed by the rock debris originating from the microbial decay constantly occurring at the colonized rocks of the springs. This process results in the leaching of silica, uranium, zinc and other metals and minerals and the formation of miniscule rock particles, resulting from the weathering of the stone (Heinen & Lauwers in press, Lauwers & Heinen 1985). The sediment from the main pipeline was found to contain considerable amounts of uranium (unpubl.), derived from living and dead bacteria, settled within the sediment, which accumulate this metal. They are also responsible for the relatively high percentages of organic matter in the samples from sediments and springs (Heinen & Lauwers in press). In samples from sediments and the rock substratum of the thermal springs, fungi had been observed frequently with light- and electron microscopy (Heinen & Lauwers 1985), but so far we failed to isolate them from the complex rockcolonizing microflora, because they were always overgrown by fast-growing bacteria. This could finally be overcome by applying the (slightly modified) method of Webley et al. (1960): About 15 ml of sterilized agar solution (1%) were poured over 0.6 ± 0.1 g of sediment in petridishes just before hardening and the plates were then incubated at 50°C. After two days, hyphae with and without sporangia began to develop on, around and from inside the rock particles from the main pipeline sediments (fig. 1). Fungal growth on the sediment from an iron tube was less and considerably slower. Because the medium contained no organic substrates, the by definition heterotrophic fungi must obtain their carbon and energy supply from low-molecular organic compounds present in the sediments. Soils in general and these kinds of sediments in particular, always contain extracellular enzymes that catalyze the degradation of organic macromolecules from dead cells and other sources. These enzymes are secreted by the microbes present in the soils and sediments and are also released by dead or dying cells together with a variety of intracellular degrading enzymes (Skujins 1967). This way compounds are provided which can serve as substrates for the molds. The fungus itself is also well-equipped to convert complex substrates with especially high activities of phosphatases, esterases, a-galactosidases, /?-glucosidases and other enzymes suitable to utilize bacterial remnants {table I).