Critical thinking skills, as well as cognitive style, are important aspects that must be considered in assessments of the learning process. This research aims to describe the critical thinking skills of the students with reflective versus impulsive cognitive styles through an explorative study on conservation and environmental knowledge learning. The benefits of this research are: (1) obtaining information about critical thinking based on reflective versus impulsive cognitive styles and (2) using the above as the basis for a learning model attentive to reflective versus impulsive cognitive styles. The research subjects were 26 students of the biology education class in 2013; 13 students had a reflective cognitive style and 13 students an impulsive cognitive style. Critical thinking skills were measured by using a test that refers to the standardization. Cognitive styles were measured by using the MFFT. Since the data from the assessments of students' critical thinking skills were non-parametric, they were analyzed using the Mann-Whitney test. The results indicate that 13 students, or 36.1%, each have reflective and impulsive cognitive styles. The results showed significantly higher critical thinking assessments in the students with reflective versus impulsive cognitive styles. It can be concluded that the critical thinking of students with reflective cognitive style is better than the students with impulsive cognitive style.