We have investigated the subunit structure of mitotic chromosomes of the acellular slime mould Physarum polycephalum, using the nuclease susceptibility of isolated mitotic nuclei as a probe. A characteristic pattern of DNA digestion products is obtained, containing approximately integral multiples of a basic 140 base pair DNA segment that resembles very closely the pattern in G2 phase nuclei of Physarum and of calf lymphocyte nuclei. These results demonstrate that during the process of chromosome condensation there is no alteration at the primary level of chromatin structure that is responsible for the characteristic DNA digestion pattern.