Using the Third International Mathematics and Science Study (TIMSS) 1999 data, this study investigates the relationship between 8th graders' mathematics and science achievement and their self-perceptions. For within-country data, there is generally a positive relationship between students' achievement and three measures of their self-perception: how much they like the two subjects, their self-perceived competence in the subjects and their perceived easiness of the subjects. However, on a between-country analysis (the unit of analysis being the country) the findings are the opposite, i.e. there is a negative relationship between self-perceptions and achievement. These findings are consistent with the findings from a previous study based on TIMSS 1995 data, even though the sample of countries is different and an additional measure of self-perception is applied. Among several possible explanations, the author suggests that this pattern may reflect high academic standards in high performing countries and low academic standards in low performing countries.