It is a commonplace in the history of science that reputations of scientists play important roles in the stories of scientific knowledge. I argue that to fully understand these roles we should see reputations as produced by communicative acts, consider how reputations become known about, and study the factors influencing such processes. I reapply James Secord's ‘knowledge-in-transit’ approach; in addition to scientific knowledge, I also examine how ‘biographical knowledge’ of individuals is constructed through communications and shaped by communicative contexts. My case study is Carl Sagan, widely discussed – amongst scientists, media professionals and publics – for his skill as a charismatic popularizer, his perceived arrogance, his political activism, and his debated merit as a researcher. By examining how aspects of Sagan's reputation circulated alongside his scientific work – rather than existing as a static context for his scientific work – I show how different forms of knowledge (biographical and scientific) influence each other as they circulate.