The plasmodium of Physarum polycephalum harbors billions of synchronized nuclei in a single cell of complex structure. Due to its synchrony and extreme size, it is used as a model to study events on a single cell level, such as cell cycle and differentiation. We show here for the first time that this model, despite its enormous size and structural complexity, is accessible to RNA interference by simple injection of dsRNA or siRNA. The targeted gene is that of polymalatase, an intracellular adapter of poly(beta-l-malate) involved in the maintenance of the synchrony and functioning as an extracellular hydrolase of this polymer. Real-time reverse transcriptase polymerase chain reaction analysis revealed that the specific mRNA was knocked down to about 10% of the original level. The suppression of a single injection lasted for approximately 14 cell cycles (144 h) and could be prolonged for any time by repeated dsRNA injections. Western blots indicated that the knockdown of RNA was paralleled by a strong reduction in polymalatase synthesis. However, a change in the phenotype of the plasmodium could not be clearly observed. In principle, the plasmodium offers an easy system for studying gene knockdown by RNA interference.