In a normally feeding insect, the taste receptors are exposed to complex mixtures of chemicals, not single compounds. We investigate the responses of neurons in the galeal sensilla of the caterpillar of Grammia geneura to mixtures of nutrient compounds at concentrations occurring in plants. Compounds that stimulated the same neuron were generally additive in their effects in binary mixtures. Amino acids that did not stimulate usually had no effect in mixtures with a stimulating compound, but glutamic acid reduced the response to serine in the medial sensillum. Nutrient compounds that stimulated different cells in a sensillum acted independently of each other. Complex mixtures of amino acids resembling samples of free amino acids from three host plants were less stimulating than expected from their molar concentrations. In host plant selection, the response from the medial sensillum is probably dominated by sucrose; unless sucrose levels are low, amino acids will contribute little to sensory input because they stimulate the same cell as sucrose. In the lateral sensillum, amino acids act independently of sugars. The limited contact chemosensory array of caterpillars seems inadequate to allow them to make fine distinctions between plants on the basis of their free amino acids.