The U.S. Next Generation Science Standards treat biological adaptation as a disciplinary core idea, but various science educators recommend its introduction at different ages. The concept of biological adaptation elicits children's reasoning about organisms, their environments, and the relationship between organisms and their environments. The purpose of this qualitative case study was to examine preschool children's thinking about organisms, their environments, and reasons connecting organisms and their environments throughout the learning experiences. The sample consisted of 53 preschool children aged three through six enrolled in two iterations of a week-long summer science camp. Data collection consisted of daily sticker-sorting assessments and individual post-camp interviews with the children. Data analysis included scoring children's sticker placements and coding reasons for placing particular organisms in particular environments. The results reveal age-, environment-, and organism-related trends in terms of both ability to correctly place organisms in their respective environments and reasons for placement of particular organisms in particular environments. Recommendations for developmentally-appropriate instruction for preschool science education and future research are provided.