Continuous exposure to colchicine was used to estimate the variation in cell cycle time between cells within suspension cultures ofDaucus carota. Observations were made of the pattern of disappearance of cells of the initially predominant ploidy levels in diploid and tetraploid cultures having markedly different aggregation patterns. Both cultures showed a similar range of cycle times, normally distributed about the culture mean. Shorter colchicine treatments, followed by regrowth in colchicine-free medium, showed that spread of cycle times in the diploid culture prevented uniform induction of tetraploidy, and that the resulting mixoploid suspensions showed a gradual reversion to diploidy during subsequent subculture.