The purpose of this research was to determine whether there were differences between male and female performance on the horizontal water level task. A sample of 18 boys and 17 girls who were in elementary school were selected. The Chi-square test of significance was used where variables and factors were statistically examined. Five factors were considered in relation to the horizontal water level task. They were (1) differences in performance on perception and prediction tasks, (2) sex differences in the ability to comprehend the horizontal water level principle, (3) sex differences in the stage of development as a result of instruction, (4) sex differences after instruction on the number of correct responses on horizontal tasks, and (5) changes within the stages after instruction. The findings indicated that there were significant differences between male and female performance on this task. Males did significantly better than females. The findings further indicated that there were differences between performance on prediction and performance on perception. Instruction made a significant difference in female performance as judged by change of stage on the prediction task. The results indicated that females could improve in their performance on this task when given instruction. / Master of Science