The synthetic copolypeptide (Lys33, Leu67)100-Orn20, modeled on some general features of the histone sequences, has been found to supercoil the DNA double helix, wrapping it into a micelle, as a result of cohesive interactions between the polypeptide hydrophobic moieties. X-ray low-angle diffraction of complexes between the polypeptide and DNA is characterized by maxima at 50, 32, and 23 A, reminiscent of the chromatin pattern. The existence of a nucleosome-like structure along the DNA is suggested by gel electrophoresis analysis of DNA fragments after micrococcal nuclease digestion, showing the presence of a fragment of about 100 basepairs (bp) long. Topological experiments on the complexes with supercoiled as well as relaxed circular DNA by two-dimensional gel electrophoresis show the presence of left-handed superhelical turns. The results are in agreement with an intrinsic propensity of B-DNA to writhe into left-handed supercoils.