The differential response of silkworm, Bombyx mori L. to phytoecdysteroid when administered at different ages of 5th instar was studied on a popular Chinese silkworm hybrid, Xinhang x Keming reared on the ‘Hu Sang 197' variety of mulberry leaves. Phytoecdysteroid, a chlolest -7 ene- 6- one- carbon skeleton (C27) from a plant source, Radyx achyranthes was administered to 5th instar silkworm per os at a rate of 2 µm per larva to different batches of silkworm at 48 h, 72 h and at the onset of cocoon spinning when a few larvae were ripe. The larval duration, larval mounting duration, cocoon characters and reeling traits were influenced by phytoecdysteroid. The intensity of influence was dependent on the time of application. The larvae treated at 48 h improved almost all economic traits significantly with a marginal increase in larval duration. In the larvae treated at 72 h, the economic traits were adversely affected but the larval duration was shortened by about 24 hours. In the larvae treated at the onset of cocoon spinning, the mounting duration was substantially reduced without much affect on the cocoon traits which would be a big benefit in commercial sericulture. The physiological significance of varied response of silkworm to phytoecdysteroid administration is discussed. Journal of Applied Sciences and Environmental Management Vol. 9(3) 2005: 81-86