Current research suggests that the difference in mathematics achievement between males and females in and beyond the junior high school grades is a direct consequence of the decision by many girls not to take advanced mathematics courses during high school. The first section of this paper indicates at least three factors influence this decision: (1) a lower feeling of competency among many females; (2) acceptance by many females that mathematics is a male activity; and (3) the feeling among many females that mathematics is not likely to be of much use in future career aspirations. Sections II, III, and IV compare the scores of males and females on an objective-referenced mathematics test and on a norm-referenced test. All of the scores reported are for fourth, eighth, and twelfth grades and all reflect the national pattern: that girls compare favorably with or higher than boys in the elementary grades, but show lower achievement levels in the upper grades. Section V offers some recommendations which might serve to alter this pattern of weaker mathematics preparation found among girls. These recommendations are addressed to teachers, counselors, administrators, and parents, and suggest specific ways of encouraging more girls to enroll in mathematics courses. (Author/MP)