This study cn Melanagromyza n sp. an insect which is thought to be a tomato pest, was carried out wiih the purpose of supplying the necessary informations to solve this problem, because is considered a problem into the tomato planting. The study was made on tomato plantings near Lomitas (Prade- ra) and in the Entomology laboratory of the Agronomy Faculty at Palmira. The study was made on some tomato plantings, one at 1.056 meters above sea level and the other at 1.270 meters above sea sea level. Various attempts to grow the insect artificially were not succes- fully. This can probably be attributed to: 1) Lack of an essential nutritive element in the diet of the insect; 2) Unfavorable environment or 3) The conditions under which the adult insect were held in cages. However, feeding larvae artificially that were brought from the field it was possible to get adult insects. But these insects did not produce eggs. Commplementing data from Oatman (9) and Essig (6) using eggs of the Agromyzidae family it was possible to estimate the life cycle of the Melanagromyza n sp. This cycle gave a minimun of 24 days, a maximun of 32 days and a average of 28 days, distributed as follows: eggs 3-6 days, larvae 7-10 days and pupas 14 - 16 days. The degre of infestation in different planting of tomatoes varied in relation to altitude. The highest porcentages of infestations ocu- rred at intermediate altitudes of 1.186 meters above sea level to 1.226 meters above sea level. The insects attacked the tomato plants for 20 - 30 days after transplanting and again at 20 - 30 days later. They prefer the base of the leaf Stem especially the lower part, 10 their posuires. The larvae ascenae or descende along the main stem of the plant and the secondary branches. In the field it was discovered that the Amaranthus spp. is a host plant. This weed was found in abundance in the zone svudied. The attack of the larva of Melanagromyza n sp. is associated with a fungus of the genero Fusarium which increase the damage to the plant. However, the Melanagromyza n sp. has a natural enemy kwnow as Syntomopus sp.; Order Hymenopiera; Superfamily Chalcidoidea; Famny Pteromalidae; as classified in the United States. The parasite Syntomopus sp. was found in 19% of the insects of the ivieiana- groroyza n sp. found in the tomatoes planting under study.