Results of a study made to determine the effect of male-female class ratio on female student class participation are reported. In spring 1980, an all-female section of an introductory math course was offered in addition to the other five mixed-sex sections at a midwestern university. It was found that women selecting the all-female section had lower math ability scores (SAT); yet, at the end of the course, these women attained higher achievement scores than women in the mixed-sex classes. Women in both groups had significant decreases in level of math-anxiety during the term, so that the greater achievement of the women in the all-female section could not be attributed to reduced level of math-anxiety. Also, returning women students--those over 25 and away from academia for at least 5 years--performed better in the all-female class. Finally, there was a lower withdrawal rate in the all-female section. Since the only difference between the all-female and mixed-sex sections was group gender, differences in class participation of women in the two environments were examined by an observer who sat in on each of the six sections to record student participation. The greatest participation was found to be in the all-female section, followed by the only mixed-sex class that had a female instructor, and an evening section with many returning students. (LP)