The objectives of the study were to (1) develop an instrument for measuring the mathematical thinking of high school students and (2) determine the effects of the class level and student's gender on the mathematical thinking of high school students in Nebraska. The subjects of the study were 239 high school students (grades 9‐12) selected from 18 Central Nebraska high schools participated in the study. The mathematical thinking instrument was developed by the researcher and administered to all subjects of the study in April 1994. Analyses of data collected indicated that significant differences in mathematical thinking of high school students existed in favour of the higher class level in all cases with the exception of grades 11 and 12. The mean score of the 11th grade students was significantly higher than that of the 12th grade students. In addition, no significant differences were found between the mean scores of male and female high school students at any class level or on any of the six characteristics of mathematical thinking dealt with in this study. The researcher recommends that more studies dealing with mathematical thinking be conducted to improve the measuring instrument in order to employ it for the discovery of students’ specific weaknesses in the development of their mathematical thinking. These studies will provide educators with guidance for developing effective curricula, methods and materials to remedy such weaknesses.