Studies of both professional science practice and children's science learning show that care is not merely ancillary to disciplinary work but a core and generative constituent of science practice. In science education research, however, students' care is often overlooked. In this paper, we describe the expression of care across two STEM classrooms (6th and 9th grade) studying biology and ecology and participating in a pen pal exchange. We analyze artifacts from the pen pal exchange as well as students' retrospective interviews and written reflections. Two ways of expressing care surfaced in students' letters: caring for guppies and caring for pen pals. We describe each form of care using examples from our data. We find that students' care for guppies and pen pals was both instrumental (in service of their investigations) and intrinsic (positioning guppies and pen pals as inherently valuable). We then connect these findings to studies of care in children's science learning and in professional science. We discuss methodological and practical implications for recognizing and analyzing how students' care manifests in classrooms and for designing learning activities that cultivate care.