Visual signals are hitherto assumed to play only a minor role in intra-specific communication in temperate anurans. However, in contrast to most species, males of the Balkan moor frog Rana arvalis wolterstorffi develop a conspicuous dorsal body coloration during the breeding season; its functional significance, however, remains to be clarified. In a first step, we quantified the colour change by measuring the spectral reflection of the eardrum, flank and back of males during immigration and the spawning climax by means of a spectrophotometer between 300 and 700 nm. We found that males change their body colour from dull brown with a more equal reflection spectrum to a bright ultraviolet (UV)-blue with strong reflection between 350 and 450 nm. In contrast, females stay dull brown. A cage experiment revealed no effect of the presence/absence of direct physical contact between females and males on the UV-blue body coloration in males. Moreover, none of the measured colour parameters, that is overall intensity, peak wavelength, UV and blue chroma, were found to correlate with the body condition index of males, indicating that body coloration is not affected by the physical condition of the male. Based on our results and data reported in the literature, we hypothesize that the UV-bluish nuptial coloration, additionally to its assumed function as fitness indicator of the males, may constitute an intra-sexual signal to prevent males from mispairing with other males in high-density mating aggregations.