The purpose of this study was to investigate and better understand preservice science teachers' (PSTs') practices when critiquing and revising 5E (engage, explore, explain, elaborate, and evaluate) lesson plans. A single instrumental case study approach was used. The study was conducted at a public university in northeastern Turkey. The participants, chosen using a convenience sampling method, were 51 second-year PSTs. They were asked to create 5E lesson plans on a given objective in a total of 20 groups of two to three. Then each group was asked to present its plans while other groups critiqued each step of the 5E lesson plans presented. Data sources were the participants' written critique forms and the lesson plans that they created in their groups. Data were analyzed using content analysis and a rubric for the inquiry orientation of each lesson plan. Findings indicated that the PSTs made both structural and procedural critiques of the 5E lesson plans. Three of the most repeated critiques were a lack of scientifically oriented questions, a lack of alternative assessment, and a lack of gathering and analyzing data. The participants adopted some critiques on the essential features of inquiry and inquiry orientation of the lesson plans, such as formulating explanations. However, they failed to address critiques on organizing data and evaluating explanations by considering alternative explanations. After the critique activity, the inquiry orientations of half of the lesson plans increased. Implications for science teacher education concerning the 5E lesson plan critique activities are discussed.