The year-round food habits of white-tailed deer (Odocoileus virginianus seminolus Rafinesque) were studied in the wet prairie-tree island habitat of the southwestern Everglades system during 1988–91. Microhistological analysis of rumen contents revealed that a single plant, swamp lily (Crinum americanum L.), constituted 40.4% of the annual diet by dry weight and occurred in the rumens of 87.7% of 39 adults (≥ 1 yr in age) and 18 fawns (< 6 mo in age). No other food among the 35 plant items identified in the rumens constituted more than 6.6% dry weight. Swamp lily was the top-ranked food by dry weight for adult females (53.3%), adult males (29.6%), fawn females (27.1%), and fawn males (31.6%). Diet, by food category (e.g., aquatic herbs, woody plants) did not differ between the winter dry season and the summer wet season. Adult females consumed more aquatic herbs (69.8% dry weight) than adult males (38.2%), and, conversely, adult males consumed more woody plants (31.9%) than adult females (18.3%). These dietary differences between adult females and males resulted from the differential use of habitats by the sexes, likely to maximize reproductive success. The diet of fawns did not mirror that of adult females. Fawns consumed more woody plants than did adult females, which probably reflected the use of tree islands by fawns for concealment to avoid predation during their first months of life.