In this research, pre-service science teachers' writings written in a biology laboratory course were examined and the development of writings skills was monitored from the critical thinking skills perspective. In this context, the preservice teachers were asked to design and build a fully enclosed ecosystem model (bottle ecosystem), collect data from these ecosystems, and report on all processes. The observational case study design was used in this research and research was carried out with fifty-two preservice science teachers from a public university in Turkey. To determine the development of preservice science teachers' writing skills, unstructured focus group interviews and document review methods were used for data collection. The content analysis method was used for the analysis of the data. According to analyses, pre-service science teachers had difficulties in reflecting their critical thinking in their writings, but it was determined that they were able to improve themselves in the process. According to the research, if an appropriate learning environment is provided, preservice teachers can reflect their critical thinking in their writings, and this can improve their science literacy.