Although both the tea-tussock moth, Euproctis pseudoconspersa STRAND and the gypsy moth, Lymantria dispar L. lay their eggs in tight cluster, the larvae of the former live in compact aggregation until they reach to the last instar, while the larvae of the latter disperse out within a few days after hatching. Laboratory experiments were carried out to clear up the effect of individual number on the survival and development of the larvae of these species.<br>Euproctis pseudoconspersa STRAND<br>The larvae which hatched out from one egg-mass were reared at 1, 2, 5, 10, 20 and 40 individuals per vessel under a temperature of 23∼27°C with the natural light condition. When the larvae were kept singly or in pairs, most of them wandered and died without feeding. On the other hand, if 5 or more individuals were reared together, they made a group and developed successfully. The larval duration seemed to increase with decreasing the number of larvae in association, because the larvae in smaller groups generally took more instars. However, the pupal duration, the weight of pupa and the fecundity did not differ significantly, unless the ill effect of overcrowding was brought about.<br>The larvae were isolated to a solitary condition at each instar until which they had been reared in mass, and were further bred under a temperature of 27.5°C with a day-length of 16 hours. At the younger instar the larvae were isolated, the higher the mortality of them tended to be. Especially, none of the larvae isolated at the 1st to 3rd instar could survive, while those isolated after the 7th instar did not die at all. The larval duration and the number of ecdysis were increased by the isolation at younger instars. However, there seemed to be no difference in the pupal duration, the head width of the last instar larva the weight of pupa and the number of eggs per female.<br>Lymantria dispar L.<br>The larvae were reared at 1, 2, 5, 10 and 20 individuals per vessel under a temperature of 23∼27°C and the natural light condition. In every culture all the larvae developed successfully. With the increase of the number of larvae per vessel, the larval and pupal durations, the weight of pupa and the fecundity decreased progressively.<br>Thus, the formation and maintenance of aggregation are essential for the larval life of Euproctis pseudoconspersa STRAND and the ill effect of undercrowding is brought about by the artificial isolation of larvae. On the contrary, when the larvae of Lymantria dispar L. which show no strong aggregation are crowded together, the adverse effect of overcrowding tends to appear.