Tested whether a single dot moving on a trajectory can be detected when it is presented amid noise dots whose movements are indistinguishable from the trajectory dot's on a frame-to-frame basis. Ss judged the presence of the signal within a 2-alternative forced-choice paradigm. Ss were shown 2 stimulus intervals of which only 1, randomly selected each trial, contained the signal. The Ss judged in which interval the signal was presented. Results showed that Ss can detect 1 dot moving on a fixed-trajectory amid moving noise dots. It was also found that detection is not simply dependent on the density of noise, but rather jointly on noise density and step size. Detection measured with many different densities and step sizes fell on a single curve when plotted against the probability of a mismatch.