We investigated the expression patterns of a granule-bound starch synthase I (GBSSI = Waxy) gene at different developmental stages of storage and non-storage organs in Amaranthus cruentus. GBSSI transcripts were strongly expressed in the middle and mid-late stages of seed development and thereafter expression decreased. In addition, this gene was expressed in all non-storage organs tested (the leaf, stem, petiole and root) and showed a tendency to increase during plant development. Therefore, our results indicate that the amaranth GBSSI gene exhibits late expression in the perisperm, and that it is expressed in both storage and non-storage tissues. We also investigated the genetic diversity of GBSSI among 37 strains of amaranth grains originating from New World. A comparison of the GBSSI coding sequence revealed an extremely high level of sequence conservation, and a single nucleotide polymorphism between the sequences of non-waxy (Type I) and waxy (Type II) phenotypes was detected. This indicates that a G–T polymorphism in exon 10 (a nonsense mutation) was a unique event in the evolution of the GBSSI gene in amaranth grains.