Sea urchin embryo chromatin is hydrolyzed by micrococcal nuclease into a series of oligomers which are multiples of a monomer (repeating unit) containing 220 ± 22 nucleotide pairs of DNA which accumulates during the initial phase of the digestion. Although the size of the chromatin monomers remains the same throughout early development, from the morula through the pluteus stage of embryogenesis, the rate and extent of solubilization of chromatin DNA by micrococcal nuclease decrease as development proceeds. Sea urchin sperm chromatin is hydrolyzed by micrococcal nuclease into a series of oligomers which are multiples of a monomer containing 260 ± 26 nucleotide pairs of DNA which accumulates during the initial phase of the digestion. Analysis of the sizes of oligomers which result from micrococcal nuclease digestion of mouse liver, sea urchin embryo, and sea urchin sperm chromatin in situ, suggests that the oligomers are nearly exact multiples of the respective monomers. These results are discussed in relation to those studies which have shown that the histone complement of the sea urchin embryo and sperm changes during development.