Black Redstart males usually have female-like (olive-brown) feathers during their first breeding season. This subadult coloration contrasts sharply with the grey and black feathers of the adults. To examine the proximate mechanisms of this phenomenon, known as delayed plumage maturation, we assessed levels of testosterone in circulating blood of subadult and adult males captured in Prague, Czech Republic. Analysis of 23 blood samples (performed by radioimmunoassay) collected during the molt, i.e., at the time of plumage development, revealed significantly higher testosterone levels in males molting to the adult color than in those molting to the subadult one. This may suggest a certain role played by testosterone in the regulation of delayed plumage maturation. However, there were no marked differences between males of different coloration and/or age during the breeding season. Analysis of 46 blood samples collected outside the molting period confirmed the seasonal testosterone pattern (spring peak and winter minimum) typical of most temperate passerines.