Measured speed discrimination for random dot stimuli that contained many different speeds in 2 experiments with 3 observers. Ss always based their discrimination on the mean speed of the stimulus; changes in other stimulus characteristics (e.g., mode) were not detected. Discrimination thresholds for the global mean speed derived from many different speeds were comparable to those obtained with stimuli in which all dots move at the same speed, suggesting that the internal noise associated with the encoding of speed information is quite high.