We documented the winter, spring and summer food habits of trumpeter swans (Cygnus buccinator) in the greater Yellowstone area (the intersection of Idaho, Montana and Wyoming) and studied the diet preference of nesting swans. Although 23 foods were detected in trumpeter swan diets during the winter, spring and summer, only 8 contributed at least 3% to the diet during any one season (Table 1). Dominant foods (over 10% of the diet in at least one season) included Chara spp. (21.7%, 4.1%, mean, SE), Elodea canadensis (11.4%, 3.1), Potamogeton spp. (32.3%, 4.2), and Potamogeton pectinatus tubers (15.7%, 3.6). Potamogeton pectinatus tubers were highly preferred by swans and tuber consumption significantly (P = 0.03) increased from winter (23.4%, 10.5) to spring (38.5%, 10.0). The primary food in summer was Potamogeton foliage which accounted for 48.2% of the summer diet. Nesting trumpeter swans significantly (P = 0.039) prefer Potamogeton spp. when it was available at feeding sites within their territories. Chara spp. was eaten in proportion to its availability, and swans avoided eating Ceratophyllum demersum and Myriophyllum exalbescens.