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Seasonal dietary variations demonstrate the importance of certain plant parts during the year. A parallel analysis of their nutritional constituents provides further information on underlying patterns of consumption of the plant parts and the relative importance of key nutrients. I studied the diets of Lemur catta (ring-tailed lemurs) and Propithecus verreauxi verreauxi (sifakas), for 9 mo over a 13-mo period in the highly seasonal tropical dry forest site of Beza Mahafaly in southwestern Madagascar. I tested dietary plant parts for nutrients—protein, free amino acids, and sugars—and for 2 potential deterrents, phenolics and tannins, using plant extracts prepared in the field. I compared consumption of nutrients and secondary compounds throughout the year and between seasons. Nutrients are balanced throughout the year. The 2 lemur species do not appear nutrient-starved in either season, though actual quantities of nutrients and contributing food parts differ for each species. Lemur catta consumes high levels of sugar throughout the year, whereas Propithecus takes in higher levels of protein. The effects of phenolics and tannins are quantitative, and they appear to deter consumption of plant parts only past a certain threshold. Sifakas consume them in greater quantities than those of ring-tailed lemurs, which appear more sensitive to their effects. Sifakas may have a higher tolerance for secondary plant metabolites, which is consistent with reports for other folivores. The overall stability of nutrients throughout the year indicates no lean period that coincides with the decline in food abundance during the dry season, though actual caloric intake probably decreases.