Interactions of herbivorous copepods with their phytoplankton food depend on the size composition of organisms in both trophic levels. A simulation model is used to analyse these size-dependent relations with the following conclusions. 1. Relative size structure of herbivores and their food is more important than total biomass of each trophic level in determining modes of transfer from plants to herbivores. In nearly all cases, in the model, food limitation affects reproduction or the first feeding stage of the nauplii. 2. No single factor emerges as predominant in determining the size structure of both populations. 3. The nature of predation on the herbivores is at least as important in determining both phytoplankton and herbivore size composition as physical or nutrient parameters. 4. The magnitude of the population of the larger herbivores such as Calanus, important as food for fish, depends on their coexistence with the smaller copepod species which control the smaller phytoplankton. 5. Stress on the system, if it affects adversely the smaller herbivores, can lead to the breakdown of the Calanus-diatom component. 6. Prediction of the population structure for both plants and herbivores may be a more attainable objective of theory and more practically important than prediction of total biomass at each trophic level.