Robert Whittaker's five-kingdom system was a standard feature of biology textbooks during the last two decades of the twentieth century. Even as its popularity began to wane at the end of the century, vestiges of Whittaker's thinking continued to be found in most textbook accounts of biodiversity. Whittaker's early thinking about kingdoms was strongly shaped by his ecological research, but later versions were also heavily influenced by concepts in cell biology. This historical episode provides insights into important intellectual, institutional, and social changes in biology after World War II. Consideration of the history of Whittaker's contributions to the classification of kingdoms also sheds light on the impact of Cold War politics on science education and educational reforms that continue to shape the presentation of biological topics in introductory textbooks today.