Purpose – The purpose of this paper is to improve the critical thinking of biology students of STKIP Hamzanwadi Selong on an environment course through the implementation of Problem-Based Learning (PBL) and Group Investigation (GI) through Lesson Study. Design/methodology/approach – This study employed a descriptive qualitative method. The subjects of the study were 73 freshmen biology students academic year 2013/2014 who were divided into two classes. The Lesson Study was conducted in 16 cycles, comprising Plan, Do, and See stages. The instrument of the study used to measure the students’ critical thinking was an essay test which covered several criteria of critical thinking adapted from SOLO Taxonomy. Findings – The implementation of PBL and GI encouraged the students to think critically through planning, arguing, stating questions and problems, and analysing and providing solutions to the surrounding environmental problems. Originality/value – This study depicted Lesson Study activity on an environment course which focused on the students’ critical thinking activity through the integration of PBL and GI. The learning was conducted through Lesson Study and was based on the environment. The crucial value of the Lesson Study was the lecturers showed more attention to the students since they wanted to know the students’ learning style and what made them learn.