Among 1598 lower respiratory tract samples corresponding to patients admitted at the San Juan de Dios Hospital, San José, Costa Rica, between november 1996 and august 1997 (The frecuency of isolation of Moraxella catarrhalis) was 4,3%. Patients with Chronic Obstrutive Pulmonary Disease (copd) had a higher risk of presenting Moraxella catarrhalis (p< 0,05) and those 60 or more years of age were more susceptibility to infection. Antibiotic susceptible tests were performed for all the strains of Moraxella catarrhalis that were obtained and they yielded ad more than 90% susceptibility to amikacin, ampicillin, cefotaxime, cephalothin and chloramphenicol . It was determined that Moraxella catarrhalis is not a primary lower respiratory tract infection agent, even though the clinical history of the patient must be considered as an important guide on the isolation of this microorganism and subsequent treatment.