The semidwarfing sd1 gene, the most common height-reducing gene in rice, is present in most modern cultivars. It was introduced into indica rice varieties in the 1960s with IR8, the cornerstone of the Green Revolution. Independently, a different sd1 allele was identified in a japonica background following mutagenesis (Mackill and Rutger 1979). The presence of sd1 can be determined only from the final height of the plant, which is scored late in development, and is influenced by environmental factors. Furthermore, sd1 is a recessive gene, and heterozygosity at the sd1 locus can be determined only by progeny testing. The development of a molecular marker for sd1 would therefore provide breeders with additional options to test for the presence of short or tall alleles rapidly and at any growth stage.