Recent research has shown that the presence of stereotype-relevant environmental cues can inadvertently bias people's judgments of others in the direction of the stereotype. The present research demonstrated analogous activation effects on self-stereotyping. In two experiments, the effects of stereotype activation on the tendencies to stereotype others and to self-stereotype were examined. Experiment 1 tested whether incidental exposure to gender-related materials might activate gender stereotypes and hence affect perception of another person. Experiment 2 investigated gender stereotype activation effects on female and male high school students' self-presentation behaviors. The results showed that incidental exposure to stereotype-relevant environmental cues increased both stereotyping and self-stereotyping tendencies. The findings were discussed in terms of their implications for understanding the basic principles of knowledge activation and application, and for reducing stereotyping and self-stereotyping.