The effect of stimulus presentation probability on response bias was analyzed in a 2-response color identification task. The design represented an attempt to examine a stimulus frequency component of presentation independently of the response frequency component. The 25 Ss responded with one hand to a green stimulus and with the other hand to either a red or blue stimulus. When the red and blue presentation ratios were shifted from 1: 5 to 5: 1 the response latencies to these colors shifted in such a way that the more frequent stimulus always yielded the faster latency. The results were interpreted as indicating that part of the effect of presentation probabilities on response bias is attributable to a pure stimulus frequency factor.