This study aimed to investigate the differences between: 1) low-ability students' critical thinking skills and high-ability students' critical thinking skills, 2) male students' critical thinking skills and female students' critical thinking skills. This research was a survey conducted in some public Senior High Schools (SMAs) in Batu, East Java, Indonesia. An essay test which examined students' critical thinking skills in biology was distributed to 245 participants. The results of the test were analyzed using a critical thinking skills rubric. Data analysis was performed using ANOVA technique. Research findings showed that 1) high-ability students' critical thinking skills were higher than low-ability students' critical thinking skills, and 2) female students' scores on the critical thinking test were higher than male students' scores. These results suggest that it is necessary to implement learning strategies which can promote students' critical thinking skills in the biology classroom.