In order to obtain data concerning the minimum size requirements for adequate tactual discriminability of the wheels and flaps airplane control knobs, five groups of 20 subjects each performed a task which required that they discriminate tactually between these two knob shapes. Subjects were allowed to view a knob shape presented in an aperture for about two seconds. Immediately following this they were required to grasp a lever containing one of the two knob shapes. The subject was to operate the lever if he thought that the knob shape sampled tactually was the same as the one viewed but was to remove his hand without moving the lever if he thought it was different. Five different knob sizes were used and each of the five groups performed the discriminative task with one of the different knob sizes. The major dimension of the knobs used varied in 14 inch steps from 1 inch through 2 inches. Each subject made 48 discriminations. Analyses were made of 1 average number of initially correct responses for each group, 2 average response time for initially correct responses for each group, and 3 average time required to make 48 correct responses for each group. None of these analyses revealed any statistically significant differences among the five knob size groups.