Bradysia sp. Frey, 1948 (Diptera: Sciaridae) damages mostly ornamentals, vegetables, and Agave spp. in greenhouses and nurseries. In Mexico, study has been infrequent, and only the identity of species of fungus gnats associated with crops of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) and forest nurseries has been reported. To contribute to knowledge of the species, an experiment was done at 24.7 ± 1.4°C, 53.9 ± 3.2% relative humidity, and photoperiod of 18:6 light:dark hours in a laboratory to determine the length of the biological cycle of the fungus gnat. The life cycle from egg to adult emergence was 25 ± 2.65 days. Eggs hatched 3.63 ± 0.5 days after oviposition. The larval stage lasted 17.19 ± 3.02 days, while the pupal stage was 2.19 ± 0.4 days. The adult stage was 2 days. This is the first report of the biology of the fungus gnat associated with lilies (Lilium L.) and its presence in the State of Oaxaca.