ABSTRACT.1Eggs of the British woodland mosquito,Aedes punctor(Kirby), were subjected to a variety of media in an attempt to determine the hatching stimulus.2It was found that continuous immersion in distilled water evoked almost no hatching (0.2%); intermittent removal from the water gave low hatching (8.9%).3Slow deoxygenation of the medium by either bacterial growth or the introduction of nitrogen produced the highest level of hatching (26.0–90.0%). Eggs ofA.punctorwere shown to hatch within a 4 h period of the oxygen concentration in the medium reaching zer