Allocation of photosynthate by phytoplankton in Georgian Bay was measured from spring through fall of 1993 to test previous models for environmental control of allocation and to assess for the first time the effects on allocation of solar ultraviolet radiation. Ultraviolet-B radiation (UV-B, 280-320 nm) at near-surface levels significantly inhibited photosynthesis (by up to 82%) under both natural and artificial light. UV-B effects on allocation to protein, polysaccharide, low molecular weight materials, and total lipids were rarely significant, but the sensitivity of photosynthesis to UV-B increased directly with allocation to protein. UV-B exposure had more influence on allocation among lipid classes, but the effects were still statistically marginal. Allocation to protein and glycolipids varied directly with silicon availability, while allocation to total lipids and glycolipids varied inversely with phosphorus availability. Previously published models for predicting protein, polysaccharide, and total lipid allocation from temperature and daylength were unsuccessful. Lipid classes showed distinctive seasonal patterns and environmental correlates, consistent with differing functional roles and (or) taxonomic associations among the major lipid classes. The allocation of photosynthate, and thus the food quality and nutrient stoichiometry of the primary producers, appeared to be under complex control that could not be precisely predicted from any simple combination of environmental variables.