The galls are structures formed of plant tissues in response to the activity of different types of organisms, especially by insects. As a consequence of an intimate relationship with their host plants, most of these insects have a very narrow host range. In this study we report the occurrence of galls on Microgramma mortoniana. Morphological characteristics and field observations are presented. The study was carried out in a seasonal semi-deciduous forest fragment, located in the São João do Oeste municipality, Santa Catarina, southern Brazil. The galls present a fusiform shape and similar indument to the rhizome, in a higher density. In the region of occurrence of the galls, a widening of the rhizome was observed, reaching an average of 9 mm (±1mm), whereas in a rhizome with normal growth the width is 4 (±1 mm). The gall is unicameral, with the following dimensions: 7 mm (±1 mm) x 2 (±1 mm). The larvae observed into the chamber are about 0,7 mm long, being representative of the family Tortricidae (Lepidoptera). In all cases, the rhizomes with galls do not presented continuous development and, consequently, the rhizome has its limit of growth defined by the occurrence of the gall. The results indicate that a greater sampling effort of fern galls in the Neotropics is needed, contributing to the knowledge about the distribution of the galls on ferns lineages.