Competition among rotifer clones has been postulated to be an important force influencing rotifer population structure. Competition was investigated experimentally using laboratory populations of Asplanchna. Clones adapted to environments separated in space and clones adapted to environments separated in time were examined. In each case, the population dynamics of the competing clones were characterized, then the intensity of competition described. Population growth rate was found to be regulated by food availability and feeding rate was closely associated with competitive ability. Competition between clones was found to be intense whether clones were adapted to environments separated in space or time. Competitive replacement of 1 clone by another took 16 generations for the spatially separated clones and 5 generations for temporally separated clones. Strong competitive interactions among clones have definite consequences for rotifer population structure. In a competitive environment a group of conspecific rotifers in a lake at a particular time should consist of 1 or a few clones, much of the clonal variability being eliminated by competitive exclusion. Strong competitive interactions should also promote strict temporal partitioning of the environment. As a result, a rotifer species in a lake should be represented by a temporal sequence of homogeneous populations, 1 replacing another through time as the environment changes. This population structure is further suggested by evidence for restricted gene flow between populations and electrophoretic studies. Considered together, these data suggest that the model of complete genetic discontinuity is the most likely structure of rotifer populations.