Experiments were conducted to evaluate the effectiveness of some weed extracts against field dodderCuscuta campestris Yunck. on alfalfa (Medicago stiva L.) and to identify and quantify the phytotoxic agents of these extracts. All concentrations of aqueous extract of every weed showed significant effectiveness on dodder when compared to the untreated plant under lath house and field conditions. Control percentages of the (0.5 g) of Bermuda grass (Cynodon dactylon) and wall goosefoot (Chenopodium murale) ranged between 83 and 96, and the same concentration caused injury to alfalfa foliage up to 43% when applied in the field. Phytotoxic agents were identified as phenolic compounds such as chlorogenic, isochlorogenic,p-coumaric acids, and scopoletin. Their quantities varied with species; Bermuda grass had the highest content (32.2 μg/g dry weight) followed by Johnson grass (Sorghum halepense), well goosefoot, and tumble pigweed (Amaranthus albus). These results might aid in screening for effective alternate approaches for controlling dodder on alfalfa planted for seeds.