Siamusotima disrupta Solis, n. sp., is a stem-boring musotimine from China. It was discovered in the stems of several Lygodium Sw. species (Lygodiaceae) during exploration for biological control agents of Lygodium microphyllum (Cav.) R. Br., the Old World climbing fern. This is the second report of a stem-boring pyraloid larva with a unique, cup-like modification of the anal segment. The larval morphology of S. disrupta is compared with that of the previously described S. aranea Solis and Yen. Observations on life history are presented; the adult has not been captured in the wild.