To maximize fitness, animals must choose food appropriate for their physiological status and dependent on the food available in the environment. Tannic acid, protein and fiber content of food affects the physiology of herbivorous mammals, so we studied the effects of these factors on food selection of Microtus fortis. We used laboratory paired-choice tests with four experimental diets: 10% protein + 2.25% fiber with 3 % and 6 % tannic acid content (foods 1 and 2, respectively), 20% protein + 2.25% fiber with 3% and 6% tannic acid content(foods 3 and 4, respectively). The four diets were presented to M. fortis in six paired combinations: food 1 with foods 2, 3, and 4, and food 2 with foods 3 and 4. and food 3 with food 4 .M. fortis ate more of the diets with 3% tannic acid than those with 6% tannic acid , regardless of protein content(4.41 vs 1.83 g; 4.00 vs 2.36 g for the low and high protein diets, respectively, P<0.001) . Further, they ate significantly more of the 6% tannic acid diet when it was coupled with 20% protein vs. 10% protein (2.36 g vs 1.83g, respectively; P<0.05). Unexpectedly, when tannic acid was low (3%): voles ate much more of the 10% protein diet than the 20% protein diet (3.21 g. vs 2.01 g, respectively; P<0.001). However, there was no significant difference between the intake of low and high protein diets when coupled with high (6%) tannic acid. Overall, Microtus fortis food intake was most strongly determined by tannic acid content, then by fiber content, and lastly by protein content (but there was no statistical difference between protein and fiber when tannic acid content was 6%).