The factors inducing male offspring and the mechanism of sex—ratio adjustment were investigated in Daphnia magna, a cyclically parthenogenetic cladoceran (Crustacea). Laboratory experiments were conducted on individual animals living in flow—through chambers, to separate the effects of various density—dependent factors. Fluctuating food levels had no effects on offspring sex when starvation was not severe. Water from crowded populations of different Daphnia species contained substances that induced up to 43% male broods. The chemically mediated crowding effect was detectable under conditions representing a low population density, implying that animals responded to their own metabolites. Daphnia species differed in their inducing effects, and in one case (water from D. pulex) strong male offspring induction was associated with growth inhibition. A short daylength was also demonstrated to induce male offspring effectively. Evidence was found to support a mechanism of sex—ratio regulation that implies a regular alternation of brood sex, resulting in a sex ratio close to 1/2, the expected optimum ratio. Even individual mothers adjusted the sex ratio of their offspring within a sequence of seven broods. No resting eggs were formed in the experiments, indicating that formation of male offspring and resting eggs are independently controlled, possibly by distinct sets of environmental cues.