ADP-glucose pyrophosphorylase (AGPase) is one of the major enzymes involved in starch biosynthesis in higher plants. We report here the molecular cloning of two cDNAs encoding so far uncharacterized isoforms (AGP S2 and AGP S3) of the potato enzyme. Sequence analysis shows that the two polypeptides are more homologous to previously identified large subunit polypeptides from potato and other plant species than to small subunit isoforms. This observation suggest that AGP S2 and AGP S3 represent novel large subunit polypeptides. agpS2 is expressed in several tissues of the potato plant, including leaves and tubers. Expression was stronger in sink leaves than in source leaves, indicating developmental regulation. In leaves, agpS2 expression was induced 2- to 3-fold by exogenous sucrose; therefore, agpS2 represents a new sucrose-responsive gene of starch metabolism. Expression of agpS3 was restricted to tubers: no agpS3 expression could be seen in leaves of different developmental stages, or when leaves were incubated in sucrose. Therefore, agpS3 represents the only AGPase gene so far characterized from potato, which is not expressed in leaves. Conversely, all four AGPase isoforms known from potato are expressed in tubers.