The dynamics of the host–parasite relationship between tomato cv. Brigadeand Egyptian broomrape is temperature-related. This relationship wasutilized for the development of an equation on the basis of thermal time (asmeasured by growing degree days, GDD, C) to predict the parasitism dynamicsof Egyptian broomrape in tomato. To obtain a reliable prediction fromthermal time values, studies based on a wide range of temperatures areessential. Four temperature-regime treatments and five levels of infestationwith Egyptian broomrape seeds were tested in a multiclimate greenhouse(phytotron) and a temperature-controlled greenhouse, respectively. Theday/night temperature regimes were 20/12 C, 23/15 C, 26/18 C, and 29/21 Cand the infestation levels were 0 (noninfested control), 1, 5, 10, and 25 mgof Egyptian broomrape seeds per liter of soil. As expected, increasingtemperature or infestation levels resulted in faster appearance and higherrate of attachments, respectively. The relation between development ofattachments and GDD was described as a three-parameter logistic curve. Inboth temperature-regime and infestation-level experiments, the developmentof attachments began 200 GDD after planting and the maximal number ofattachments was recorded 800 GDD after planting. A significant reduction inthe aboveground biomass of the tomato plants due to increased Egyptianbroomrape biomass was recorded only for the 26/18 C and 29/21 C day/nighttreatments and the three highest infestation levels (5, 10, and 25 mg L−1 soil). The ability to predict the start of parasitism canbe used to develop a climate-based system for Egyptian broomrape controlwith herbicides.