The consumption of plants by the zoophytophagous bug Macrolophus pygmaeus Rambur (Heteroptera : Miridae) in relation to the presence or absence of prey is monitored for 7 days. Tobacco plants enriched with 15N, a stable, nonradioactive isotope of nitrogen, are used for characterization. In the absence of prey, the relationship between time and 15N content (which equates to plant feeding) is linear, with a constant daily accumulation of 15N; however, if prey are available, the mathematical relationship becomes curvilinear. From day 1 to day 5, feeding on plants is independent of the consumption of prey. The rate of 15N accumulation decreases in the presence of prey after day 5, whereas the number of prey eaten remains unchanged.