Whether gender differences in performance using computer software are due to sex stereotyping or gender differentiation in the programs was investigated in two studies. An adventure game, "King and Crown," with all male characters, and a gender neutral game, "Honeybears," were played by 26 female and 26 male 11- and 12-year-olds in Milton Keynes (United Kingdom). Both games were essentially route-planning tasks with similar features. In a second study, a new version of "King and Crown," called "Pirates," was compared to "Honeybears" in a study with 24 female and 24 male 11- and 12-year-olds in Milton Keynes. Considered together, results of these studies provide a clear illustration of the way characteristics of the software can exert an effect on the performance of girls. While the performance of boys was relatively unaffected by the software, that of girls was far superior when playing "Honeybears." Further studies will examine the expressed preferences of boys and girls for the two games. (SLD)