The influence of the parasitic angiosperm Orobanche aegyptiaca on the growth and shoot/root allometry of tomato plants was studied in two experiments. In the first, the density of infection was manipulated, with host and parasite biomass being measured 56 d after planting (d.a.p.). In the second, multiple harvests were made from 14 to 91 d.a.p. at one level of infection (20 mg seed dm−3). The first experiment demonstrated an approximately linear reduction in host biomass up to 10 mg seed dm−3 soil, beyond which there was no further reduction. The parasite also depressed shoot/root ratio, which changed prior to any decrease in total biomass. These trends were maintained in the second experiment, becoming more pronounced with time. Significant reductions were observed in host biomass compared with that of uninfected controls from 42 d.a.p., following the emergence of the parasite above ground, and corresponded with the onset of lower relative growth rates (RGRs) in the infected plants. Infection also influenced components of RGR: there was a stimulation of leaf area ratio (LAR) and a depression of unit leaf rate (ULR). The data are discussed with respect to the influence of other parasitic weeds on host growth.