This report summarizes a number of exploratory and pilot studies undertaken to investigate the relationship of individual differences among college mathematics students to discovery and expository instruction, and to Personalized Systems of Instruction (PSI). Research in education has often attempted to demonstrate the superiority of one type of instruction when compared with another. This research is based on the idea that no single treatment is satisfactory for all students, but that different pupils do best with different types of instruction, depending on the individual student's characteristics. Matching instructional treatments with student characteristics to maximize learning is called Aptitude-Treatment-Interaction (ATI). Significant ATI's were obtained in the reported studies with three different aptitude variables. Field dependence-independence, general reasoning, and locus of control appear to be the most promising variables for future research, particularly in mathematics education. A complete report of each study is provided in the appendices. (Author/MP)