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Empirical data providing evidence for a colimitation of an herbivore by two or more essential nutrients are scarce, particularly in regard to biochemical resources. Here, a graphical model is presented, which describes the growth of an herbivore in a system with two potentially limiting resources. To verify this model, life-history experiments were conducted with the herbivore Daphnia magna feeding on the picocyanobacterium Synechococcus elongatus, which was supplemented with increasing amounts of cholesterol either in the presence or the absence of saturating amounts of eicosapentaenoic acid (EPA). For comparison, D. magna was raised on diets containing different proportions of S. elongatus and the cholesterol- and EPA-rich eukaryotic alga Nannochloropsis limnetica. Somatic and population growth of D. magna on a sterol- and EPA-deficient diet was initially constrained by the absence of sterols. With …