The ultrastructure and distribution of intercellular junctions in 5 human ependymomas of the fourth ventricle are described. The most common form was the simple apposition of cells with a cleft, about 200 A wide, between their cell membranes. The zonula adhaerens was predominant in cells near microvilli which were present along a widened intercellular space. Another specialized intercellular junction, the zonula occludens, was met less frequently than the zonula adhaerens. The zonula occludens occurred mainly near the zonulae adhaerentes. Occasionally, a cytoplasmic plaque as seen in the zonula adhaerens was closely applied to the cell membrane in the zonula occludens. The zonulae adhaerens and occludens were frequent in well differentiated ependymomas but much less so in poorly differentiated ependymomas. The zonula occludens was often absent in the poorly differentiated tumour. The distribution of the zonula occludens is discussed on the physiological basis of a marked difference in function of the intercellular communicating system of normal and malignant cells.