Chinese hamster ovary (CHO) cells plated on agar form two classes of colonies; those which increase continuously in diameter and those which become arrested in outward growth. All colonies continue to increase in mass and thickness as demonstrated by computer-assisted analysis of time sequence photographs of several thousand colonies and by examination of histological sections. Colonies which shift from a monolayer to a mounded morphology at fairly large colony diameter (greater than or approximately 1 mm) continue to increase in diameter. Colonies in which mounding occurs at smaller colony diameter (less than or approximately 1 mm) cease to increase in diameter but continue to increase in thickness, as demonstrated by histological examination and by computer-assisted analysis. Rapid cell division occurs at the edge of all colony classes, as shown by the distribution of mitotic figures. In arrested colonies these dividing cells must move toward the colony core to compensate for dying cells. Necrotic cells are found as a discrete zone at the air-colony interface in all cases for CHO cell colonies growing on agar. As such, necrosis is probably due to limitations in nutrient diffusion upward from the agar rather than oxygen diffusion downward.