This study is concerned with the identification and subunit localization of ribosomal proteins in Dictyostelium discoideum. The characterization is based on the resolution of ribosomal proteins by various methods of electrophoresis. 34 and 42 unique proteins were identified in the 40-S and 60-S ribosomal subunits respectively. The total mass of proteins in the 40-S subunit was 746,100 daltons and 981,900 daltons in the 60-S subunit. The molecular weights of individual proteins in the 40-S subunit ranged from 13,200 to 40,900 with a number-average molecular weight of 21,900. The molecular weight range for the 60-S subunit was 13,800--51,100 with a number-average molecular weight of 23,400. The 80-S ribosome contained 78 proteins, two of which were lost upon its dissociation into subunits. All the proteins of the 40-S and 60-S subunits could be identified individually in a 80-S map as well as in unfractionated proteins from whole cells. Purification of ribosomes in high-ionic-strength buffers resulted in non-specific loss of the various proteins from the 40-S and 60-S subunits. In addition, the undissociated ribosomes contained about 10 acidic proteins in the molecular weight range 50,000--100,000, which were retained after washing the ribosomes in high-salt buffers. They were found in polysomes, run-off ribosomes and could also be identified in the 40-S subunit after dissociation.