Gender differences have been reported in the United States in the areas of mathematical reasoning favoring males and verbal abilities favoring females. Results from the National Assessment of Educational Progress Science Assessment also have consistently revealed small gender differences favoring males. This paper reports gender differences in Costa Rican elementary and secondary schools and discusses some of the possible mechanisms related to the development of these differences in the Costa Rican context, especially those pertaining to the scientific area. Gender differences are reported for mathematics, science, and Spanish. The grade levels were 4th, 6th, 7th, 10th, and 11th. The data for this analysis were collected as part of larger studies on evaluation of the teaching of the subject matters executed by the Research Institute for the Improvement of Costa Rican Education from 1982 to 1986. Results indicated that at the fourth grade level, there were no significant differences in any subject matter. The boys had significantly better achievement in science and mathematics than did the girls for 6th, 7th, 10th, and 11th grade levels. On the other hand, the girls surpassed the boys in the same grade levels in Spanish, except for 10th grade where no significant sex differences were found. (CW)