Three experiments tested a signal-detection theory (SDT) model of visual search (e.g., as described in J. Palmer, C. T. Ames, & D. T. Lindsey, 1993). In Experiment 1, participants searched for a 0 degrees line among distractors at (a) 30 degrees; (b) 1/3 at 30 degrees, 2/3 at 50 degrees; (c) 1/3 at 30 degrees, 50 degrees, and 70 degrees; and (d) 1/3 at 30 degrees, 2/3 at 70 degrees. The SDT model predicts improved performance in the more heterogeneous conditions, as some distractors are more discriminable from the target. In contrast, in Experiment 1 human performance degraded in the more heterogeneous conditions (c and d). In Experiment 2, sparser displays improved the performance of the SDT model. In Experiment 3, search for theta degrees among homogeneous theta + 20 degrees distractors was compared with search for theta degrees among theta+/-20 degrees distractors. Performance in the latter condition was often worse, relative to performance in the homogeneous condition, than predicted by the SDT model; however, this depended greatly on the identity of the target.