Critical thinking is an intellectual process that actively and conceptually conceptualizes, implements, analyzes, synthesizes, and evaluates information gathered or generated from observation, experience, reflection, reasoning, or communication, to guide beliefs and actions. The purpose of this study is to describe the characteristics of the students' critical thinking level in solving valid and reliable microbiological problems. The type of this research is descriptive research using qualitative approach. Subjects in this study were 4 students of Biology Education, Universitas Lambung Mangkurat academic year 2014/2015, taken from high, medium, and low category based on the students’ score of microbiology course. Data analysis techniques used include data reduction, data presentation and draw conclusions. The results of this study indicate that based on its characteristics, at a less critical level, the students are able to understand the problem and prepare the plan of completion by fulfilling the reasoning element, the critical level enough for the students to understand the problem, to prepare and implement the problem solving plan by fulfilling the element of reasoning. Characteristics at a critical level are being able to understand problems, develop plans, implement problem-solving plans and re-examine problem-solving outcomes by fulfilling some logical elements except the elements of reasoning and inference and at a very critical level students are being able to understand problems, plan, implement problem-solving plans and review the problem-solving results based on the elements of reasoning. Students who take the course of microbiology certainly have different characteristics based on a written assignment that has been given. Keywords— critical thinking, problem solving