Mean periods from egg seeding to larval jumping (EJ-period) of the melon fly, Dacus cucurbitae, were compared between mass-reared (MS:F₅₀) and newly-colonized strains (NS:F₁₃) at three egg densities. The EJ-period of MS was significantly longer than that of NS at each density: 7..6, 10.1 and 13.1 d in MS, and 6.5, 6.7 and 8.2 d in NS, respectively, at densities of 50, 300 and 3, 000 eggs/450 ml of medium. The proportion of MS and NS larvae which had matured by the 6th day after egg seeding amounted to 75.0% and 77.8%, respectively, and those matured by the 8th day attained 98.8% and 99.5%, respectively, at an egg density of 300/450 ml. the difference between the percentages of the two strains was insignificant, indicating that the period from egg seeding to larval maturation (EM-period) of MS was almost equal to that of NS. These results suggest that the prolonged EJ-period of MS was caused not by a lengthened EM-period but by delayed larval jumping behavior after maturation.