Abstract.1. The response curves for the photoperiodic induction of the sexual forms (oviparae and males) differ significantly in an English clone of the pea aphid Acyrthosiphon pisum Harris. Male production is sharply peaked. The late summer scotophases (dark periods) that initiate male production are 1.0–1.5 h shorter than those that initiate ovipara production; the induction of males, but not oviparae, virtually ceases at scotophases longer than 12 h. This disparity suggests that there are two photoperiodic clocks.2. All ovipara‐producing aphids switch spontaneously to virginopara production part‐way through the progeny sequence, irrespective of photoperiod. This may confer the ability to overwinter partheno‐genetically under favourable climatic conditions. Since males are always born last in the progeny sequence this possibility would be pre‐empted unless the terminal male sequences were suppressed by long winter scotophases.3. The role of the photoperiodic response in determining the seasonal phenology was tested in the field by exposing a succession of laboratory‐reared aphids to natural photoperiods, in late summer and autumn. Adult males and oviparae developed synchronously in early October, the difference in critical scotophase compensating for the late appearance of males in the progeny sequence. A large part of the ‘civil twilight’ is photoperiodically active.4. Clones propagated outdoors by isolating aphids in each generation from the terminal sequence of virginoparae did not survive the whole winter of 1985/86. The chances of survival were reduced by severe weather and by the inability of the photoperiodic system to ‘turn off’ the production of early born oviparae which are,