Aims: To characterize the biofilms in an interim wet storage system (CLAB) for spent nuclear fuel. Methods and Results: Planktonic cells and biofilms were analysed with fluorescence microscopy, and scanning and transmission electron microscopy. The organisms in the biofilms were filamentous, consisting of sheaths with series of rod-shaped cells in them. Planktonic cell populations ranged between 1.4 x 10(3) and 5.2 x 10(3) ml(-1), correlated with the system configuration, and was inversely correlated with total organic carbon (TOC) levels. Analysis of 16S rDNA indicated that a potentially novel Meiothermus sp. was dominant in the CLAB biofilms. A Meiothermus-specific probe was designed, and statistical analysis of fluorescence in situ hybridization results confirmed that Meiothermus sp. composed up to 98% of the biofilm. The low TOC levels (2-55 mug l(-1)) in the system combined with elevated water temperatures (ca 36degreesC) mimic the natural environments of Meiothermus sp. as well as the recommended conditions for isolating Meiothermus sp., thus supporting the results of the 16S rDNA analysis. Conclusions: A possibly novel Meiothermus sp. is the predominant genus found in the CLAB biofilms. Significance and Impact of the Study: TOC levels are often kept low to protect industrial systems from biofouling. However, the present study shows that, rather than protecting industrial systems against biofouling per se, maintaining low TOC levels might still result in biofilms, dominated by new, unknown bacterial species.