Nuclei of spermatozoa of the sea cucumber Holothuria tubulosa contain the five somatic-type histones plus a sperm-specific histone H1 and a unique basic protein phi 0, which is related to H1 in amino acid composition. No proteins of the High Mobility Group (HMG) type have been detected. The structure of this chromatin has been probed nuclease digestion. Its behaviour is anomalous, since two distinct fractions of chromatin are recovered from these spermatozoa, which differ either in the presence or absence of the sperm-specific proteins H1 and phi 0. This heterogeneous distribution is not found in conventional materials, such as calf thymus or chicken erythrocytes. Proteins H1 and phi 0 are not uniformly distributed and may be localized in special regions of chromatin. Fragments containing long stretches of nucleosomes lacking both proteins can be recovered. At the same time, the chromatin fractions which contain these two proteins are shown to be less soluble. When an extensive digestion of chromatin is carried out yielding only nucleosomes and small oligomers, the H1 and phi 0 proteins redistribute themselves on chromatin, the two proteins acting in a cooperative fashion in this process. Cross-linking experiments carried out in whole cells indicate a proximity of phi 0 and H1, whereas no crosslinks have been detected between phi 0 and any of the four nucleosomal histones. The phi 0 protein may thus play a role similar to histone H1 and be only loosely associated with nucleosomal histones, but contribute to the structuration of chromatin during spermiogenesis.