Mammals can have either generalized (mixed) or specialized diets. We expected swamp wallabies (Wallabia bicolor) to have mixed diets, and predicted a negative relationship between the selection of a food type and its relative availability (negative frequency dependence). We collected data on diets in a native Eucalyptus forest where the relative availability of food types (defined as 5 plant functional groups: ferns, forbs, monocots, shrubs, and trees) had been altered by timber harvesting. A comparison of diets between individuals living in 2 habitat types (unharvested forest and 5-year-old regenerating areas) showed that in both habitats forbs were the major dietary component, although moderate amounts of shrubs and monocots also were consumed. Trees and ferns were eaten less at unharvested sites, and more at 5-year-old sites. Nonmetric multidimensional scaling followed by a multiresponse permutation procedure demonstrated a substantial difference in diet composition between the habitats (multiresponse permutation procedure: A = 0.20, P < 0.001), but when analyzed using an index of diet selection, the difference was smaller (A = 0.05, P = 0.04). Three alternative analyses demonstrated negative frequency dependence in many cases, a result generally consistent with a mixed feeding strategy. With the exception of tree foliage, selection was positively correlated with the relative availability of at least 1 other food type, and largely uncorrelated with 3 forage quality variables (nitrogen, water, and dry matter digestibility). Additional data at a finer resolution and in different seasons are required to test the generality of these conclusions.