Understanding the diet of the vulnerable red-bellied lemur ( Eulemur rubriventer) is essential for preventing its extinction. Previous E. rubriventer research focused on populations in Ranomafana National Park, located in the southeast part of the island, making research in other forest locations a priority for the understanding of dietary breadth and constraints in this species. My research questions consider whether protein is the nutritional driver for foraging choices in E. rubriventer, how fiber influences cathemeral activity, and seasonal variability in day and night time dietary items. My research team and I identified the plant and animal species that composed a single female's diet in the Ankadivory region, near Tsinjoarivo. A team of three to four people conducted daytime follows for 30 days during the lean season. For six of these days, three teams worked in shifts to cover the 24-hour cycle. Additionally, there were six 24-hour abundant season data collection days following the same procedure. Dietary food items were sampled for laboratory analysis. Analysis of plant samples for macronutrient content was used to quantify intake of available protein, non-protein energy (carbohydrates and fat), and fiber. The first hypothesis I tested, protein is the limiting macronutrient in E. rubriventer diets and it drives foraging choices, was not supported. The study subject was able to meet her daily requirements of protein in the lean season, and fat was the most constrained macronutrient between seasons. The second hypothesis I examined, fiber intake is a good predictor of cathemerality of E. rubriventer, was also not supported. Fiber was consumed all year at relatively constant amounts (NDF CV: lean season = 12.7 %, abundant season 9.2 %) and fiber consumption did not correlate with increased nighttime feeding. The third hypothesis I tested, nighttime diet composition differs from daytime diet composition, was supported. The E. rubriventer consumed dietary items higher in fiber during nocturnal feeding bouts (leaves in the lean season and flowers in the abundant season). This project provides information crucial for a more comprehensive understanding of food selection by E. rubriventer. The 24-hour diet information revealed that, for cathemeral animals such as E. rubriventer, only looking at daytime diet consumption can lead to an inaccurate diet profile.