The paper presents a study that aimed to reveal the process quality descriptors of an argument-based inquiry approach. Process quality signifies whether a teacher is a better or unsatisfactory implementer of a version of in-class science inquiry. An experienced middle school science teacher was the participant. Based on the initial analysis completed by the Reformed Teaching Observation Protocol, it was observed that the teacher was a high-capacity implementer of the argument-based inquiry approach. Through episode-based analysis, discursively regulated indicators of the process quality were identified. The teacher adopted a flexible instructional agenda for the high process quality by welcoming students' everyday social languages. The teacher also combined low interanimation (collecting alternative utterances) and high interanimation (eliciting alternative statements) of ideas. The teacher avoided an early conceptual consensus by pressing the students to cope with a discussant who could not be persuaded easily by unwarranted claims. The teacher played the devil's advocate role by making the students' cognitive contradictions explicit and tangible. Before the experimenting phase, the teacher intentionally elaborated on the student-proposed research variables and guided them to evaluate other groups' experimental outcomes. Educational suggestions are presented about teacher noticing for enhancing the process quality.