Recent studies have revealed that β-amylases (EC 3.2.1.2) located in plastids participate in endogenous starch degradation. On the other hand, many plants possess a respectable amount of extraplastidic β-amylases. Because of their existence outside of the plastids, it has been believed that extraplastidic β-amylases do not contribute to starch degradation in plants. Here, we investigated the role of the major extraplastidic β-amylase of the radish (Raphanus sativus) (RsBAMY1) in starch metabolism. Amylase activity was elevated in the growing taproot of the radish. During the elevation of amylase activity, the starch content decreased, but the contents of soluble sugars such as maltose, glucose, fructose and sucrose increased. In the radish taproot, RsBAMY1 protein accumulated in the primary cambium and anomalous cambium of the storage parenchyma. The starch granules, however, were found just inside the cambium and adjacent to the anomalous cambium. When the RsBAMY1 gene was overexpressed in Arabidopsis, the plants contained lower starch contents than the wild-type plant. These results suggest that the extraplastidic β-amylase may affect the starch metabolism in some plants.