H1 phosphorylation has been studied through the precise nuclear division cycle of Physarum polycephalum. The number of sites of phosphorylation of Physarum H1 is very much larger than the number of sites reported for mammalian H1 molecules which is consistent with the larger molecular weight of Physarum H1. At metaphase all of the Physarum H1 molecules contain 20-24 phosphates. Immediately following metaphase, these metaphase-phosphorylated H1 molecules undergo rapid dephosphorylation to give an intermediate S phase set of phosphorylated H1 molecules containing 9-16 phosphates. Progressing into S phase newly synthesized H1 is phosphorylated and eventually merges with the old dephosphorylated H1 to give a ladder of bands 1-20. By the end of S phase or early G2 phase, there is a ladder of bands 1-16 all of which undergo phosphate turnover. Further into G2 phase the bands move to higher states of phosphorylation, and by prophase all of the H1 molecules contain 15-24 phosphates which increases to 20-24 phosphates at metaphase. These results support the proposals that H1 phosphorylation is an important factor in the process of chromosome condensation through G2 phase, prophase to metaphase.