From January to March 2006, powdery mildew was observed in onion (Allium cepa L.) foliage (cv. Mercedes) grown at the University of Puerto Rico-Agricultural Experiment Station in Juana Diaz, P.R. Climatic conditions prevailing during this period were low precipitation, temperatures ranging from 20 to 31 °C, and low relative humidity. First symptoms appeared 75 days after sowing. Leaves showed white to gray powdery patches that coalesce affecting the entire area. Symptoms were observed on young and mature leaves. A visual scale was used to estimate disease severity in the field ranging from 1 to 5 (1 = 0%, 2 = 1-25%, 3 = 26-50%, 4 = 51-75% and 5 = 75-100% of plants affected). Over all, plant damage observed in the field ranged from 26 to 50%. Leaf samples were collected and examined under the microscope. Powdery growth on the leaf consisted entirely of conidia and conidiophores singly or in groups (156 χ 10 pm), that emerged through the stomata. Two different types of conidia were observed, a primary lanceolate conidia (48 χ 10 pm) and secondary cylindrical conidia (52 χ 15 pm). On the basis of morphological characteristics (i.e., conidiophore and conidia) the pathogenic fungus was identified as Leveillula taurica (Lev.) (teleomorph of Oidiopsis sicula). The sexual stage of the fungus was never observed in the field. To our knowledge, this is the first report of powdery mildew of onion caused by L. taurica in Puerto Rico.