Bacterial strain LV43 was previously isolated from a floating microbial mat located in Movile Cave, the access point to a chemoautotrophically based groundwater ecosystem in southern Romania. This gram-negative, rod-shaped organism grows autotrophically through the oxidation of thiosulfate and sulfide, but it does not grow heterotrophically. Strain LV43 grows over a pH range of 5.0 to 9.0, with an optimum near 7.5 at 28 degrees C. The pH of the medium decreased from 7.5 to 6.5 during growth on thiosulfate. Carbon isotope fractionation values for strain LV43 were within the previously reported range of fractionation values for the overall floating microbial mat in Movile Cave and were similar to values reported for chemoautotrophic sulfur-oxidizing strains of Thiobacillus neapolitanus and Thiomicrospira sp. The 16S rRNA gene sequence of strain LV43 was determined, and phylogenetic analysis indicated that strain LV43 was most closely related to Thiobacillus thioparus and the uncultured bacterial strain Strip2, which is represented by a 16S rRNA clone obtained by direct PCR from the Stripa research mine in Sweden. This identification of strain LV43 is supported by its G+C content of 62%, which is within the range reported for strains of T. thioparus. Fluorescently labeled polyclonal antibodies specific for strain LV43 were used to locate and enumerate this strain at different locations in Movile Cave and in nearby surface-water and groundwater sources. Strain LV43 was found only at aerobic, neutral-pH sites within the cave. Strain LV43 was also found outside Movile Cave in surface waters and in groundwater believed to intercept the same sulfurous aquifer as Movile Cave.