This study used a case study, one of the qualitative research methods, with the aim of developing guidance materials based on the argumentation-based science learning (ABSL) approach for concepts in the 6th grade unit "Sound and Its Properties" and to examine students' argumentation levels and conceptual understanding. The activity sheets completed by the students before and during the implementation process and the semi-structured interviews conducted after the implementation process were used as data collection instruments. A rubric developed by Erduran et al. (2004) was used to address one of the sub-research questions of the study to determine students' level of argumentation. The semi-structured interview data were subjected to descriptive analysis. Responses to conceptual questions were coded as correct, partially correct, incorrect or blank. These categorizations were then summarized in tables, including representative student statements. The results of this study indicate that the students' level of argumentation during the first activities was initially at level 1. As the activities progressed, there was an increase in the number of level 2 arguments, with some activities reaching level 3. However, no level 4 or 5 arguments were produced. The developed materials had a positive effect on students' conceptual learning.