Heliothis subflexa (Guenée) (Lepidoptera: Noctuidae) is a monophagous insect specialized in feeding on fruits of the genus Physalis (Solanales: Solanaceae). In Mexico this fruitworm is present in all producing tomatillo areas but at very different levels of infestation. The present study aimed to provide data on the damage percentages caused by H. subflexa along an altitudinal transect ranging from 660 to 2,300 m asl. Evaluations were carried out biweekly on 8 plantations located at various altitudes (m asl) at various locations in the state of Morelos. By random sampling of tomatillo fruits in 5 locations per plot, the percentage of damage was estimated. The results obtained indicate that this species is very damaging at all altitudes in the range of 660 to 1,320 m asl, whereas at altitudes progressively higher than 1,320 m asl, populations become progressively less dense and progressively less damaging, so that at the higher altitudes the pest is not considered to be a phytosanitary problem.