In the adder (Vipera berus) multiple inseminations during the spring mating season have been demonstrated, with fertilization taking place several weeks after mating activities have ceased. Subsequent sperm competition in the female reproductive tract can result in within-season mixed paternity. Prolonged sperm storage between mating seasons has been suggested as another mechanism of sperm competition in the adder. Competition between new and old spermatozoa would obstruct efforts to reveal reproductive strategies that correlate observed behaviour to paternity in natural populations. In the present study, controlled breeding experiments with adders were performed and DNA fingerprinting was used to determine paternity among all offspring of a total of 15 litters. The offspring of 11 large females, that had most probably also mated in a previous season, were sired exclusively by enclosure males. This result and earlier empirical data support the hypothesis that long-term sperm storage, over one or several reproductive cycles, does not occur in the adder.