Allelopathic activity of Cuscuta epithymum L. (CVCEY), Cuscuta campestris Yuncker (CVCCA), Phelipanche ramosa (L.) Pomel (ORARA), Phelipanche mutelii (Schultz) Reuter (ORARM) and Phelipanche spp. (PHESS) on germination and initial development of test plats of Lactuca sativa L. cultivar 'Great Lakes' was studied under laboratory conditions. It was found that, water exracts of the parasitic weed species in concentrations 0.4, 0.8, 1.6, 3.2, 6.4 and 12.8% w/v have a relatively high inhibitory effect on the seed germination of test plants. The inhibiting rate of parasitic weed species from family Convolvulaceae ranges from 6.24 to 100.0% and for the species of family Orobanchaceae from 42.1 to 100.0%. Parasitic weed species from family Orobanchaceae (Ph. ramosa, Ph. mutelii and Phelipanche spp.) showed a considerably stronger allelopathic effect (GIaverage 17.9), as compared with the applied concentrations of water exracts of species from family Convolvulaceae (C. epithymum and C. campestris) (GIaverage 22.7).