Five non-allelic dull (du) mutants have been isolated and characterized as low amylose mutants in rice (Oryza sativa) endosperm. Synthesis of amylose is controlled by Waxy the gene that encodes granule-bound starch synthase. We previously showed that the du1 and du2 proteins may encode general splicing regulators which affect the expression level and splicing efficiency of Waxy(b) (Wx(b)), an allele that has a single-base mutation in the 5' splice site of intron 1. Here we report that the du3 mutant has also reduced the splicing efficiency of Wx(b) pre-mRNA. Furthermore, map-based cloning revealed that Du3 encodes the rice homolog of a cap-binding protein 20 kD subunit (CBP20), a component of the heterodimeric nuclear cap-binding complex (CBC) playing a role in pre-mRNA splicing, RNA nuclear export, and nonsense-mediated decay. Transient expression of green fluorescent protein (GFP)-tagged Du3 proteins in rice protoplasts resulted in localization to the nucleus showing a speckled pattern similar to the pattern displayed by other splicing factors.