After the United States Congress passed the Water Pollution Control Act of 1948, biologists played an increasingly significant role in scientific studies of water pollution. Biologists interacted with other experts, notably engineers, who managed the public agencies devoted to water pollution control. Although biologists were at first marginalized within these agencies, the situation began to change by the early 1960s. Biological data became an integral part of water pollution control. While changing societal values, stimulated by an emerging ecological awareness, may (...) explain broader shifts in expert opinion during the 1960s, this article explores how graphs changed experts’ perceptions of water pollution. Experts communicated with each other via reports, journal articles, and conference speeches. Those sources reveal that biologists began experimenting with new graphical methods to simplify the complex ecological data they collected from the field. Biologists, I argue, followed the engineers’ lead by developing graphical methods that were concise and quantitative. Their need to collaborate with engineers forced them to communicate, negotiate, and overcome conflicts and misunderstandings. By meeting engineers’ expectations and promoting the value of their data through images as much as words, biologists asserted their authority within water pollution control by the early 1960s. (shrink)