The nucleolar DNA of Physarum polycephalum has been differentially labelled with 3H-thymidine and the structure of the nucleolar chromatin investigated by digestion with micrococcal nuclease. Nucleolar chromatin which had been labelled in G2 phase of the cell cycle and then digested before mitosis had an identical DNA repeat length to main band DNA (165 +/- 5 base pairs) but there was a definite indication that the rate of digestion was faster for nucleolar DNA than for main band DNA. Nucleolar chromatin which had been labelled in G2 and the label chased through mitosis into G2 phase of the next cycle showed an identical DNA repeat length to main band DNA and was also digested at the same rate. We conclude that nucleolar chromatin, at least 25 per cent of which is maximally transcriptionally active in G2, has a nucleosome-like structure.