Arachnidism can be manifested either through the skin or systemically. The genus Loxosceles is typically associated with wide necrotic skin lesions, in some cases accompanied by a generalized affection. This is a clinical case of a 16-month-old male, with an ulcer-necrotic lesion in the inferior abdomen associated to a spider bite. In addition, the patient had fever, was vomiting, and was in a general state of malaise, with plateletpenia, leukopenia and anemia. Summarized are the pathogenic processes, clinical findings and recommended treatment methods. Something worth noting is the importance of the clinical history since the spider is rarely found and therefore difficult to identify.