The purpose of this study was to explore cognitive process used by selected preservice elementary teachers on basic types of problems involving number sense. The sample was selected from students in six intact entry-level mathematics sections of a course populated by preservice elementary school teachers. In the beginning of the semester, the instrument used to collect data was the Number Sense Test (NST). The score from the NST was analyzed. Six participants were randomly selected for individual interviews from students who scored in the top 10% and six participants whose scores ranked in the bottom 10% on the NST to be interviewed. All students interviewed in this study do so voluntarily and completed data collection tasks during the Spring 2002 semester for the study. According to the interview data, the data indicated that items including fractions were more difficult for low ability students than decimals or whole numbers. The low ability students tended to use rule based methods more often than the high ability students. Furthermore, the low ability students relied on standard written algorithms more than reflecting number-sense-based methods. High ability students applied number sense twice as often as low ability students. (Contains 10 tables.)