Choice behavior in detection tasks demonstrates that perceptual decision-making not only depends on sensory evidence, but also on choice biases: the tendency to choose one alternative over the others independently of the sensory evidence. To assess whether choice biases pervade other perceptual decision scenarios, we asked humans to perform a simple common perceptual discrimination task with two symmetric alternatives. We found that participants did not choose the two alternatives equally often, which is consistent with the occurrence of choice biases, but also with the occurrence of sensory biases. To test these possibilities, participants performed the task reversing the mapping between perception and the category of the alternatives. With this simple manipulation, participants reversed the frequency of choosing the two alternatives, which supports that the biased choice behavior was caused by biased sensory evidence. We also found consistent estimates of the sensory biases using a task with two asymmetric alternatives. Perceptual decision-making in simple tasks, thus, might be entirely based on the representation of sensory information.