This is a first case of mycotic aneurysm due to Streptococcus constellatus in a 58 years old male patient with diffuse large B cell lymphoma. A mycotic aneurysm is a rare but life threatening and potentially fatal sequela of bacteraemia. A 58-year male patient initially presented with sepsis after 7 days of chemotherapy for non hodgkin's lymphoma. No clear focus of sepsis was found. Blood culture grew Streptococcus constellatus. He received appropriate antibiotic and discharged well. He represented after 5 days with excruciating intrascapular and chest pain. This patient with haematology malignancy had radiologically, surgically and histologically confirmed mycotic aneurysm that involved ascending aorta which contained pseudoaneurysm. Patient was treated excision of the infected tissue and aortic graft, and long term antibiotics. This rare case highlights that the high clinical suspicion in patients presenting with possibly mycotic aneurysm is a critical tool in addition to perfect surgical treatment and medical treatment with adequate antibiotics. J Microbiol Infect Dis 2017; 7(4):213-216