Prototheca zopfii, a eukaryote that divides by multiple fission, was investigated to determine how growth rate controls daughter cell number. The macromolecular composition, cell size, and number of nuclei per cell were determined in cultures during balanced growth in various media. Cellular mass, ribonucleic acid (RNA), deoxyribonucleic acid (DNA), carbohydrate, and nuclear number increased as positive linear functions of growth rate, whereas nuclear ploidy remained constant with a value of 0.098 pg of DNA/nucleus. The ratios of RNA to protein, protein to mass, and carbohydrate to mass were unaffected by growth rate, whereas the ratios of DNA to protein and RNA to DNA could be expressed as curvilinear functions of growth rate, the former negative and the latter positive. The dependency of normalized gene dosage (DNA/protein) on growth rate appeared as a distinguishing feature of multiple fission. Determination of the normalized rates of protein and RNA synthesis revealed that both increase linearly with growth rate. It is concluded that Prototheca zopfii may exist in a number of physiological states which are characterized by a unique size and macromolecular composition and which are dictated by growth rate.