4 experiments using the visual search paradigm were run to examine mechanisms of feature integration. Experiments 1, 3, and 4, using different array sizes, showed that search for a conjunctively defined target is relatively fast and exhaustive when items in the display are spaced apart, but search is slow and self-terminating when the items are clumped together. Experiment 2 showed that these effects of density do not arise when the target is defined by a unique feature. On the basis of these findings and previous findings with iElusory conjunctions, we propose that two mechanisms are used for feature integration. A fast mechanism, called the coarse location mechanism, integrates features on the basis of coarse location information coded with the initial registration of the features. This mechanism works only when objects are spaced apart. A second, slower mechanism, is used when objects are clumped together.