Litargus tetraspilotus LeConte, 1856 was collected feeding on Oidium sp. (Fungi, Ascomycota, Erysiphaceae) associated with fruit trees. This is the first time L. tetraspilotus is recorded in Brazil, totaling three species of Mycetophagidae for this country. This study aims to provide a complementary description of this species based on new characters and to present information on its life cycle under laboratory conditions and fluctuation in population in the field. During the period of inventories between July 2004 and August 2006, about every fifteen days, a total of 565 specimens of L. tetraspilotus were collected, with the highest abundance found on citrus plants, with values differing significantly between the two years. The population levels differed between the seasons; spring had the greatest abundance and autumn the least. There was a significant positive correlation of L. tetraspilotus abundance with rainfall and relative humidity. Mycetophagidae, as well as other mycophagous families of Brazilian coleopterans, are barely studied, warranting further future studies of their bioecology and systematics.