Abstract. 1. Regulation of adult reproductive diapause and seasonal polyphenism was studied in two populations of the comma butterfly, Polygonia c-album (L.) (Nymphalidae), from Stockholm, Sweden, and Oxford, England.
2. In the univoltine Stockholm population short and long constant day-lengths (in the range 12–22 h) at 20°C were ineffective in averting the production of the dark, generally diapausing, morph, but the non-diapausing light morph could be produced if daylengths were increased during larval development. The procedure was especially effective with increases from 12 to 22 h, but the tendency was the same with a more realistic increase, from 18 to 20 h.
3. In the partially bivoltine English population a critical constant day-length for morph and diapause induction was found between 12 and 18 h. Decreasing daylengths above the critical daylength early or late in larval development resulted in production of the diapausing morph.
4. These results suggest a system for environmental control where day-lengths which increase throughout the larval period indicate an early date before summer solstice, meaning that there is sufficient time for a second generation of offspring to reach the hibernating adult stage before winter. The ecological significance of the results is discussed.