Temporal changes in the biomass of rotifer plankton were examined in a shallow hypertrophic lake during 1 month (April/May), when the predatory rotifer Asplanchna brightwelli develops. The abundance of herbivorous rotifers was regulated by predation from A.brightwelli and from the copepod Acanthocyclops robustus. The densities and fecundity rates of Keratella cochlearis and Keratella quadrata were negatively related with the biomass of predators. Stomach analyses showed that Asplanchna fed selectively on reproductive females of K. cochlearis, reducing the fecundity of this species. Predators induced longer caudal spines in K. quadrata, which were negatively related to the fecundity of this rotifer, suggesting a reproductive cost associated with spine production. In contrast, spine length of K. cochlearis was not related to predators, but to temperature. These results showed that predators can reduce rotifer densities through increasing mortality and through decreasing rotifer fecundity rates regardless of phytoplankton biomass. We also show that morphological defences of K. cochlearis and K. quadrata are induced in different ways.