Chemical signal is one of the primary channels of sensory input for zooplankton. Chemicals released by zooplankton individuals can affect their behavior, including migration, feeding, predator defense, growth, reproduction, and so on. Rotifers are adequate model animals for chemical ecology studies in aquatic system, with a rich literature on inducible defense and sexual production. However, studies associated with the effects of chemical signals on population competition have not been reported. Brachionus calyciflorus and Brachionus angularis are both common freshwater rotifer species. B. angularis, who has a lower threshold food concentration because of its smaller body size, would out-compete large body sized B. calyciflorus when the resource is limited. The present work examined the effect of crowding signals on population growth of B. calyciflorus and B. angularis. Culture medium conditioned by rotifers was used as the crowding signals and two crowding levels were set for each rotifer species. Population growth experiments of the two rotifers were carried out at an initial density of 1 ind./mL in 5 ml and 20 ml medium for B. angularis and B. calyciflorus, respectively. Regardless of the treatments, B. angularis reached its maximum population density at 100 ind./mL after 23 days, while it took B. calyciflorus only 3 to 8 days to reach its maximum population density, which was far lower than that of B. angularis. Both B. calyciflorus and B. angularis showed a higher population growth rate (r) and lower maximum population density (K) under the crowding signals of B. calyciflorus. However, neither B. calyciflorus population nor B. angularis population was significantly affected by the crowding signals of B. angularis. The maximum population density of B. calyciflorus was inhibited by its own crowding signal (from 9.124 ind./mL to 4.564 and 3.448 ind./mL under low and high crowding signals, respectively), leading a 50% decreasing of the carrying capacity K of the population. That was probably because of the inhibitory effects of autotoxins. Meanwhile, mixis ratios of B. calyciflorus populations fluctuated between 40% and 80%, and showed no significant differences among groups, because B. calyciflorus was sensitive to population density and mixis ratios were high at all these population densities. Because population growth of B. angularis was obviously affected by its own crowding signals, growing to a higher population density, and its threshold food concentration was lower, as a result, exclusion of B. calyciflorus by B. angularis may become quicker during the competition process. These results indicated that crowding signals may play an important role in the competition outcome between different rotifer species.