1. Unrestrained brown bullhead catfish (Ictalurus nebulosus) were trained to turn toward the higher concentration when two concentrations of a mixture of amino acids were simultaneously flowed over the cutaneous taste epithelium on the right and left maxillary barbels. 2. The minimum detectable Weber fraction was dependent upon the stimulus solution concentration with smaller Weber fractions being detected at weaker concentrations. Brown bullheads were capable of detecting a concentration difference of 0.3 Weber fractions at the highest stimulus solution concentration and 0.1 Weber fractions at the lowest. 3. Variations in the time-of-arrival of the stimuli at the two barbels, greater than those occurring during normal testing, did not alter the values of the minimum discernable Weber fraction. This indicates that spatial rather than temporal differences in concentration were the basis of these responses. 4. Anosmic bullheads were as sensitive to concentration differences as intact fish, indicating that discrimination was made using the taste receptors on the barbels. 5. These experiments show that bullheads can compare the intensity of a taste stimulus at two distinct receptor fields and thus define a gustatory chemical space. This ability is a requirement for tropotactic orientation to chemical gradients and suggests that such a mechanism may be important in nature.