Abstract The gene encoding granule-bound starch synthase (GBSS), which determines the presence of amylose in reserve starches, has been isolated from a wild-type and from an amylose-free potato by using a potato GBSS cDNA. From the analysis of five genomic GBSS clones, isolated from the wild-type potato, it is shown that GBSS is a single copy gene in potato. GBSS messenger RNA was shown to be present in a number of different tissues, but was most abundantly found in stolons and tubers. Southern blot analysis of the GBSS genes from both potato genotypes revealed that the amylose-free mutant, which lacks GBSS activity and protein, does not contain a large structural lesion in the GBSS gene. The GBSS messenger RNA was even found to be present far more abundant in the mutant than in the wild-type potato.