We investigated whether the sexes differ in science performance before they make important course and career selections. We collected teacher-report data from a sample of children from the Twins Early Development Study (TEDS) assessed at ages 9, 10 and 12 years (N greater than 2500 pairs). In addition we developed a test of scientific enquiry and administered it to a sub-sample of TEDS (n = 1135; age = 14 years). We found no evidence for mean sex differences in science performance assessed by teachers, or by our test of scientific enquiry, although boys were somewhat more variable. At a time when adolescents are making important course choices, girls are performing just as well as boys. (Contains 3 figures and 2 tables.)