Abstract.Embryonic diapause induction in the Senegalese grasshopper,Oedaleus senegalensisKrauss (Orthoptera: Acrididae), is influenced both by the photoperiod and the temperature experienced by females. High temperatures (40°C) and long photoperiods (LD 14:10h), which characterize the beginning of the rainy season in the Sahel, cause non‐diapausing eggs to be laid. Lower temperatures (25°C) and shorter photoperiods (LD 12:12h), which occur at the end of the rains, result in the production of diapausing eggs. At 30°C and constant photoperiods,O. senegalensisexhibited a long‐day‐short‐day response with critical photoperiods of c. 13 h andc.20 h, only the former value being of ecological significance. The photoperiodically sensitive stages to diapause induction in females occurred from the fifth stadium onwards. Temperature also affected the coloration of both nymphs and adults. Dark‐black and pale‐white individuals were produced by low (25°C) and high (40°C) temperatures respectively, whereas an intermediate temperature (30°C) produced individuals which were greyish brown. These results are discussed in relation to the ecology