Crowding increases the tendency of macromolecules to aggregate and phase separate, and regulated crowding contributes to subcellular organization and stress response. To explore the effect of crowding in a well-characterized model cytoplasm we developed methods to concentrate the macromolecule components of Xenopus egg extracts without changing small molecules. Egg cytoplasm contains a high concentration of glycogen that serves as an energy store for early development. When crowding was increased 1.4x, the egg cytoplasm de-mixed into two liquid phases of approximately equal volume, one of which was highly enriched in glycogen. Glycogen hydrolysis prevented and reversed de-mixing. Quantitative proteomics showed that the glycogen-rich phase was enriched in proteins that bind glycogen, participate in carbohydrate metabolism, or are part of very high molecular weight complexes. The glycogen-depleted phase was enriched in ribosomes, endoplasmic reticulum (ER) and mitochondria. Smaller soluble proteins were approximately equipartitioned. Glycogen is usually observed in aggregates in intact cells and recent work suggested a role for phase separation in its localization. Our results show that glycogen particles can spontaneously de-mix and suggest that de-mixing may be regulated by crowding.