All students beginning either the two-year National High School or the one-year Preliminary Year program in Papua New Guinea in 1980 undertook a test of formal operational thought at the beginning of their programs. At the end of the programs they undertook the same test again. Piagetian cognitive levels obtained were compared with grades in science courses. Results showed: (1) considerable numbers of students at a “transitional” level on each occasion, (2) low correlations between cognitive level and science grades, and (3) significant development in levels of cognitive thought during the National High School program. It is suggested that a knowledge of the cognitive level of students could be of particular value to teachers at this stage.