The 20S proteasome is the proteolytic complex that is involved in removing abnormal proteins, and it also has other diverse biological functions. Its structure comprises 28 subunits arranged in four rings of seven subunits, and exists as a hollow cylinder. The two outer rings and two inner rings form an alpha7beta7beta7alpha7 structure, and each subunit, alpha and beta, exists as seven different types, thus giving 14 kinds of subunits. In this study, we report the primary structures of the 14 proteasomal subunit subfamilies in rice (Oryza sativa), representing the first set for all of the subunits from monocots. Amino acid sequence homology within the rice family (alpha-type: 28.9-42.1%; beta-type: 17.2-31. 9%) were lower than those between rice subunits and corresponding orthologs from Arabidopsis and yeast (alpha-type: 49.2-94.5%; beta-type: 34.8-87.7%). Structural features observed in eukaryotic proteasome subunits, i.e., alpha- or beta-type signature at the N-termini, Thr active sites in beta1, beta2 and beta5 subunits, and nuclear localization signal-like sequences in some alpha-type subunits, were shown to be conserved in rice.