A monitoring programme was developed from July 1992 to June 1993 using light traps in a plantation of Eucalyptus urophylla in the municipality of Tres Marias, State of Minas Gerais, Brazil. A total of 437 species of the Lepidoptera order was collected and divided into the following groups: primary pests: 11; secondary pests: 18; species without defined importance to eucalyptus culture: 65; and non-identified species: 343. These four groups had 1295.35, 24.40, 127.20 and 1030.55 individuals per light trap, respectively, with a total of 2477.50 Lepidoptera individuals collected per light trap during this period. Most frequent primary pests were Psorocampa denticulata (Notodontidae), Stenalcidia grosica (Geometridae) and Eupseudosoma aberrans (Arctiidae) which were constant species (present in more than 50% of the collections) with a higher number of individuals during the driest and coldest periods, except P. denticulata, which showed more individuals during the hottest and most humid periods of the year. Faunistic analysis of this area showed a possibility of outbreaks of these primary pests, and mainly of P. denticulata.