Three experiments studied the effects of category structure on the development of categorization automaticity. In Experiment 1, participants were each trained for over 10,000 trials in a simple categorization task with one of three category structures. Results showed that after the first few sessions, there were no significant behavioral differences between participants who learned rule-based versus information-integration category structures. Experiment 2 showed that switching the locations of the response keys after automaticity had developed caused a similar highly significant interference, regardless of category structure. In Experiment 3, a simultaneous dual task that engaged executive functions did not interfere with either rule-based or information-integration categorization. These novel results are consistent with a theory assuming separate processing pathways for initial rule-based and information-integration category learning but a common processing pathway after the development of automaticity.