With the objective of investigating sex-typed interests as possible causes of differences in mathematics achievement between the sexes, the study made use of longitudinal data from the Growth Study begun at E. T. S. in 1961. Growth in mathematics achievement as measured by STEP Math and SCAT-Q was compared with changing interest patterns as reflected in certain biographical questionnaire responses. At Grade 5 there were no differences in achievement but thereafter the boys pulled ahead, while parallel differences emerged in the percentage perceiving mathematics as interesting and as likely to be helpful in earning a living. (Author)