This study investigates preservice science teachers' informal reasoning and scientific habits of mind about hydroelectric power plants. The sample of the study consists of 587 preservice science teachers who studied in four different universities in the Eastern Black Sea region of Turkey. Data were collected through a questionnaire that consists of four open-ended questions regarding a hydroelectric power plant construction and the scientific habits of mind scale. Content analysis was used to analyze the questionnaire. The same data were then subjected to quantitative descriptive analysis. In the analysis of quantitative data, the Jamovi program was used. Independent samples t-test was used to determine the relationship between scientific habits of mind and informal reasoning. Results indicated that the preservice science teachers mostly used ecological-oriented informal reasoning mode, and counterarguments were the least created argument component. Preservice science teachers' the least used informal reasoning mode was social-oriented. Informal reasoning quality of students with high scientific habits of mind score was found to be high, but no significant difference was found between the total number of informal reasoning modes and scientific habits of mind scores. In line with the results, implications were provided for preservice science teacher education.