Necrotic Arachnidism by Zorocrates guerrerensis First Case Reported in Mexico The spiders (filo Arthropoda) occupy seventh place in global species richness. The genus Zorocrates Simon, consists of endemic species in Mexico. This arthropod used for feeding or defense, glandular secretions to manipulate their prey (including human), affecting their state of health or causing the death, by what are considered species of medical importance. We present the case clinical female 19 years old, resident of the Mexico City, who came to the Parasitology Laboratory, National Autonomous University of Mexico (UNAM), commenting that three days before, sitting on his bed, felt pain in the left external tibial region at the level of the third half, discovering a large insect, repelling it immediately; minutes later, there are burning, and erythema in the area of reference, which evolves with formation of vials, which spontaneously bursts. Then presented erythematous, itchy, painful plaques, reason for the requested medical attention. The ulcerative lesion studied with slow progress until the cicatricial process. Responsible arachnid is presented to the laboratory until the end of the treatment, which is identified as Zorocrates guerrerensis, species located in areas of the Mexico City (bedrooms, bathrooms, patios, zotehuelas, cisterns and gardens). There are no cases reported in Mexico and the world, about accidents caused by this species. It is suggested that the injury is due to the mechanics of the bite and not by the toxic effects of its secretion, which has not yet been characterized and that cannot be ruled out, along with other components, as head of the pathophysiological manifestations.