In this study, an instrument, Using Mathematics in Science Self-efficacy Scale (UMSSS), was developed in order to determine preservice science teachers' self-efficacy toward the use of mathematics in their lessons. Data gathered from 250 preservice science teachers were used for Exploratory Factor Analysis (EFA) and Confirmatory Factor Analysis (CFA). The factor analysis revealed 3 factors: Self Perception of Mathematics (SPM), Utilization of Mathematics (UM), and Mathematical Skills (MS). The cronbach alpha coefficient for the overall instrument was computed 0.88. The analyses resulted in the development of a three-factor scale of 18 items that was shown to be valid and reliable. At the same time, this instrument is also the first original instrument developed for determining the preservice science teachers' self-efficacies toward the use of mathematics in their lessons. (Contains 7 tables and 3 figures.)