Farnesol and farnesyl methyl ether at concentrations of 10−3 and 10−4 M and linolenic acid (used as control compound) at a concentration of 10−3 M in the culture medium significantly inhibited the molting of Trichinella spiralis larvae in vitro. Moreover, farnesol at 10−3 to 10−4 M, farnesyl methyl ether at 10−3 to 10−5 M, and linolenic acid at 10−3 M inhibited the development of the larvae so that they remained coiled. These effects are regarded as non-specific and not related to the juvenile hormone activity of the test compounds. However, the inhibition of development of male copulatory appendages among those larvae that molted, by farnesol at 10−3 and 10−4 M and by farnesyl methyl ether at 10−6 and 10−7 M, is considered to be specific inhibition of morphogenesis attributable to the juvenilizing effects of these compounds. Two possible interpretations of the observed effect are presented.