A class of acidic nuclear phosphoproteins has been isolated throughout the mitotic cycle and at two points during differentiation in the slime mold Physarum polycephalum. The electrophoretic profiles of these proteins are reproducible and unchanging throughout the mitotic cycle, but reproducible changes occur during differentiation. The proteins are rapidly synthesized after mitosis, and their molecular weights range from 34,000 to 88,000. The proteins rapidly incorporate [32P]orthophosphate, and the content of alkali-labile phosphate increases 20 percent during the period after DNA synthesis. The proteins comprise 6.5 percent by dry weight of nuclear material while the DNA comprises about 5.5 percent. These acidic nuclear proteins may have a role in control of gene activity.