The absence of nucleosome-like structures from purified nuclei of the primitive dinoflagellate Prorocentrum micans was demonstrated by three means. i) Electron microscopy revealed mostly thin, smooth 6-nm nucleofilaments in chromatin incubated at various ionic strengths and either fixed in 0.1% glutaraldehyde or unfixed. No "beads-on-a-string" structure was found. ii) Analysis of nuclear proteins showed that low amounts of basic proteins were present (basis proteins: DNA less than 0.1), the two major one with molecular weights 12 000 and 13 000 and that histones characteristic of eucaryotes were absent. iii) Digestion of the nuclei with micrococcal endonuclease of DNase I did not result in partially digested DNA fragment repeats. Only about 10% of the bulk of the nuclear DNA was digested by micrococcal endonuclease. The high molecular weight of the remainder suggests particular protection against this type of nuclease. In the light of these distinctive nuclear features, we discuss the evolutionary position of the dinoflagellate protists with respect to the procaryotes and eucaryotes.