The role of nutrition in providing optimal broiler growth and intestinal health is essential, especially during stress or disease challenge. Feed enzymes are useful for improving performance of poultry, particularly when nutrition, management, or health status is not favorable. The objective of the following experiments was to evaluate the effect of dietary phytic acid (PA) and phytase on the performance and intestinal health of birds that were vaccinated with a live coccidial oocyst vaccine. For each experiment, half of the chicks were spray-vaccinated at day-of-hatch with Coccivac®-B and grown out in floor pens with ad libitum access to diets formulated to meet Cobb nutrient recommendations. In the first experiment, birds were given one of three diets that included different levels of a PA solution to obtain dietary PA levels of 0.74, 0.87, and 1.12% for low, medium, and high PA diets, respectively. In the second experiment, two levels of PA were included to obtain dietary PA levels of 0.75 and 1.05% for low and high PA diets, respectively. In addition, phytase was added over the top to half of the diets at 1000 FTU/kg, resulting in four diets: low PA without phytase, low PA with phytase, high PA without phytase, and high PA with phytase. Live performance parameters including body weight, body weight gain, feed intake, feed conversion, and mortality were measured as well as tibia ash (experiment 2) and indicators of small intestinal health including morphology, apparent ileal amino acid digestibility (IAAD), and pH (experiment 2). The results presented here indicated that giving broilers vaccinated against coccidiosis a medium level of PA was detrimental to feed intake, body weight gain, and it induced necrotic enteritis (P ≤ 0.05). Adding phytase on top of nutritionally adequate diets did not improve performance (P ≥ 0.05), but did improve (P ≤ 0.05) apparent IAAD and morphology of the small intestine, especially in younger birds. In addition, it has been determined that important considerations in diet formulation also can include the phytate: protein as well as calcium: total phosphorus ratios, as these may critically affect how phytate impacts bird health and performance. / Ph. D.