Insect induced galls are not known from bryophytes. Here we report the first occurrence of such galls from thalli of a neotropical liverwort, Monoclea gottschei subsp. elongata (Marchantiophyta: Monocleaceae) from Peru. This is also the first report of animal-induced galls formed in modern thalloid liverworts. The gall-inducer is a species of the family Agromyzidae (Diptera). The galls are swellings, but are otherwise indistinguishable from intact thalli as their surface is neither ornamented nor sclerotized. The histology of the galls, however, suggested that abnormal cell growth and some differentiation occurred in the parenchymatous cells of the thalli during gall formation.