The effects of photoperiod and temperature on the rate of larval development in L. dubia were investigated on a population in S. Sweden. In the field, life-cycle duration was generally 3 years. Eleven combinations of photoperiod and temperature were tested experimentally. The growth of later larval instars (except the final one) was retarded at LD 13:11 at all temperatures (15°, 20° and 25°C), while LD 19.3:4.7 and LL produced a higher growth rate which was positively correlated with temperature. Experiments performed at 20°C and LD 16:8 and 17.7:6.3 produced different results at different times of the year. A very weak long-day-induced delay occurred in the penultimate instar. The larvae spent their last winter in the final instar. When this instar was reached in constant or decreasing day-lengths, the subsequent development was retarded. During the first third of the instar, the degree of the retardation strongly increased with photoperiod in the interval LD 13:11 to 19.3:4.7 and the difference between long- and short-day responses increased with temperature from 15° to 25°C. The degree of developmental arrest in a certain photoperiod was also dependent on previous changes in photoperiod. During the middle period of the instar, development was extremely slow in most groups. The last part of the final instar was of short duration and was unaffected by light conditions. These final-instar reactions are most probably instrumental in achieving a high degree of synchronization in an advanced stage within the instar by retarding the most advanced larvae more strongly than less advanced ones. In the summer (late June, July) long days retard the development in the final instar more than in earlier instars, later on (late Aug., Sept.) short days retard mid-instar development much more than early-instar development, causing an accumulation of larvae in early midinstar. Temperature affects the effectiveness of the mechanism. The latest larvae to reach the final instar (mid-Sept.) are slowed down by low temperatures and hibernate in early intermoult stages. An increase in day-length during the final (or late penultimate) instar could stimulate development and rapidly lead to emergence. This happened in spring, when previous exposure to low temperatures supported this reaction. The advanced starting position within the final instar is considered to be an important contribution to the earliness of the emergence (early June).