AbstractParasitism byEuplectrus plathypenaeinhibits larval–larval ecdysis inTrichoplusia niby injecting nonparalytic factor(s) into the host during the stinging process via the parasitoid's ovipositor. The parasitized host moves freely, feeds, and gains weight prior to the time of the normal ecdysis but does not molt. Parasitoid development is not required for the expression of molt arrest in the host. Parasitism during the first three fourths of the larval stadium results in molt arrest. Arrestment of molting is independent of 20‐hydroxyecdysone and juvenile hormone. The arrestment factor(s) affect the epidermal tissue of the thorax and abdomen in ligated hosts without apparent interaction from other areas of the body. Cuticle and epidermal tissue of parasitized insects do not show signs of apolysis or ecdy