The goal of the present study was to investigate the relation between sex, social class as indexed by parental education level, and performance on three different types of cognitive tasks among two year old children. It was expected that social class would be related positively to superior performance on all the tasks for girls but unrelated for boys. The subjects for this study were 48 white children aged 27 months. A trained woman administered the first two tasks to each child. These were an Embedded Figures Task (EFT) and a two choice discrimination task. The third task, vocabulary recognition and vocabulary training, was administered during a home visit made within a week after the administration of the other tasks. The findings clearly support the hypothesis that parental education is related to performance on cognitive tasks among two year old girls but not among two year old boys. The possible reasons for this are: (1) girls may be biologically more homogeneous than boys due to girls' general developmental maturity relative to boys, and (2) social class has a stronger influence on the way mothers treat their daughters than on the way they treat their sons. (KJ)