A variety of 2,2-diphenylpropionate derivatives with an amino substituent were synthesized and their effects on larval growth of the silkworm, Bombyx mori, were examined by dietary administration. Of the compounds tested, 3-(4-ethylpiperazin-1-yl)propyl 2,2-diphenylpropionate (3) caused significant prolongation of the larval period. Studies on the structure-activity relationship indicate that a piperazine ring and the bond distances between the nitrogen atom and the ester group were important for this activity. Treatment of compound 3 delayed the increase of ecdysteroid titers in the hemolymph by 3 days compared with that of the control, which correlates with the delay in molting.