Subjects were required to indicate whether or not a probe word belonged to one of a variable number of categories which were held in memory. The semantic similarity among the categories was varied as was the instance dominance of the probe word. Comanipulation of these factors was expected to determine whether category similarity affects between-category shift or within-category search. However, the data suggested that category similarity and instance dominance are additive factors. Instance dominance apparently affected the time to encode the stimulus word and possibly influenced a decision stage; whereas, category similarity ostensibly affected operations involved in the search stage of processing. Semantically similar categories required less search time than semantically dissimilar categories. It was suggested that while dissimilar categories had to be accessed successively prior to search, subjects were able to consolidate similar categories so that categories were simultaneously accessed and searched in parallel. © 1974 Academic Press, Inc. All rights reserved.