Resource protein content and toughness strongly influence food selection and can have opposite effects on energy acquisition, growth, and fitness in many organisms. Few studies have experimentally tested the interactive effects of these parameters on food selection especially in aquatic plant feeders that are thought to have a lower protein diet. We investigated food preference in a pond dwelling tadpole (Rhacophorus arboreus) when offered the choice between foods that differ in toughness and in protein content (three levels each) in a laboratory cafeteria experiment. Food protein level and food toughness significantly interacted on food choice. The amount of food removed increased with food protein level but decreased with food toughness. In line with the pattern reported in some terrestrial herbivores, food toughness exerted larger effect (almost twice) than did resource protein content on food choice. Tadpoles discriminated foods of different protein levels, but they no longer distinguished between medium and low protein foods at high toughness. Low protein-soft food was preferred as much as the high protein hard food, but whether tadpoles gained an equivalent amount of energy remains unclear. These results suggest trade-offs between protein gain, harvestibility, and digestibility may exist. Food toughness strongly influenced tadpole food choice and may be a direct consequence of the strength of their mouthpart to penetrate the food materials. Resource toughness could be a factor used as a proximate cue for determining food quality in tadpole foraging strategies.