Members of theDinopbyceae are characterized by having permanently condensed chromosomes throughout the cell cycle. At interphase the chromosomes appear to have bands perpendicular to the long axis of the chromosome with a periodicity of 127 nm. Each band is composed of 2.5 nm fibers and 9.0 nm granules coiled into a helix around a central core of 9.0 nm fibers. Chromosome uncoiling has been correlated with the uptake of3[H]-thymidine. As chromosomes enter the uncoiling phase of the cell cycle they appear less dense and reveal a number of fibrous extensions. At later stages chromosomes completely uncoil into elongate fibers 127 nm in width. Chromosome unwinding corresponds to the peak in the uptake of3[H]-thymidine. Chromosomes observed on either side of the peak possess the typical interphase banding. This study demonstrates, for the first time, the fine structural details of chromosome uncoiling during a specific phase of the cell cycle. A new model of the Dinoflagellate chromonema has been derived from this study.