In order to investigate the food chain and energy balance of the chemolithoautotrophically-based ecosystem of the sulfur spring in Movile Cave as a model system for extraterrestrial life, a first sampling campaign was done. Microbial diversity and activity were analyzed by MPN-enumeration methods and microcalorimetry, resp. In addn., a speciation of the inorg. sulfur compds. by HPLC and IC techniques was performed. Metabolic activities were predominantly connected with thick microbial mats floating on the water surface of the cave. These mats showed an aerobic heat evolution of about 1200 mW/g and contained about 500 mmol/g elemental sulfur. In contrast, other samples collected from cave water, sediment and rock exhibited only activities of maximal 40 to 60 mW/g and contained only up to 2.5 mmol/g elemental sulfur. As the main primary producers aerobic and facultatively anaerobic sulfur oxidizers were identified at high nos., occasionally exceeding 107 CFU/g. Methylotrophic bacteria were present in all samples at up to 106 CFU/g, indicating the important role of C1 metab. for the cave ecosystem. Although reduced sulfur species were biol. oxidized to sulfuric acid, the pH values of the samples ranged from 6.5 to 8.2 due to the high buffering capacity of the cave walls, which consisted mainly of limestone. Surprisingly, not only neutrophilic but also extremely acidophilic bacteria were detected. Sulfate reducers were present in both aerobic and anaerobic zones. The data presented suggest a close interdependence of sulfur oxidn. and redn. as well as carbon dioxide and other C1 compd. metab. in the most biol. active zone of the cave ecosystem, i.e. the floating microbial mats.