SUMMARY.A method is described whereby ephippia (sexual eggs plus protective membranes) are experimentally induced at low food levels (≤ 0.05 mg spinachD. magna‐1), high female culture‐densities (≤ 0.4D. magnaml‐1) and in short‐day photoperiods (L:D<12:12, 12°C). The density‐dependent ephippial response was related to an increased encounter rate between females at the higher densities. External metabolites had no significant effect on ephippia production. Ephippia were formed in the second generation after exposure to short day‐lengths. Five clones from the same population exhibited genetic variation in their critical photoperiods for induction of sexual reproduction (≥= 10% ephippia). There was evidence of a genetic difference in photoresponse between two populations ofD. magna.The evolution of timing mechanisms for ephippia production are discussed in relation to the predictability of en