White cottony stem rot disease agent (Sclerotinia sclerotiorum) was isolated from the pith of infected cucumber (Cucumis sativus L. var. Loona) plants growing inside plastic-houses in the Jordan Valley. The pathogen was identified to the species level by the polymerase chain reaction using specific primers. The pathogencity of the pathogen was confirmed and then subjected to chitinase- producing actinomycetes as biological control agents. Out of the 70 different actinomycetes isolates collected from 14 different locations in northern Jordan, 40 isolates (57%) showed chitinase activity on crab shell chitin agar (CCA) media. On the basis of chitinase activity on this medium, chitinolytic isolates were divided into three groups: 14 highly (Ch1), 12 moderately (Ch2), and 14 low (Ch3) with average range of (8.3-4.7), (4.3-3.7), and (3.3-2.3) mm of clearing rings around the colonies, respectively. Additionally, such isolates were able to inhibit mycelial growth of the pathogen and thus recognized into another three groups: 13 strong (Ant1), 13 moderate (Ant2), and 14 low (Ant3) antagonists inflecting reduction in the linear growth of the fungal hyphae to (45.7-32), (31.3-22.7), and (22.3-3.7) mm reduction, respectively. High levels of chitinase activity of the isolates Ma3 (8.3mm), Ju1 (7.7mm), and Sa8 (7.7mm) of clearing rings corresponded with the magnitude of their antagonistic activity against mycelium growth with average range equals to 45.7, 44.3, and 40.7mm, respectively. Those isolates exhibited fungicidal activity against sclerotia of S. sclerotiorum. On the other hand, isolates Na5, Aj3, and Aj2 which produced no chitinase, showed fungistatic effect only. The isolates Ma3, Ju1, and Sa8 also significantly reduced the incidence and severity of the disease on cucumber plants under laboratory and greenhouse conditions.