The ultrastructure of prematurely condensed chromosomes (PCC) in BK cells and the CHO metaphase chromosomes (the PCC inducer) were studied with cell fusion technique and SEM. The methods for preparing complete specimens for SEM, were used to eliminate artifact as complete as possible, so the natural states of PCC ultrastructure, especially the ultrastructure of the early, mid, and late S-PCC, were observed for the first time. In the three phases of S-PCC multiple double strand helical structure were revealed, on the chromosome segments of BK cells. On the surface of CHO metaphase chromosomes which may induce PCC may regularly arrayed short hair-like loop-protrusions consisting of helical chromatin fibers were observed.