This article introduces the little-known contributions of Alfred Binet to comparative psychology. While Binet’s contributions to intelligence testing are well known and widely discussed in introductory psychology and history of psychology texts, his contributions to comparative psychology are largely forgotten. To acquaint textbook writers and students with Binet’s contributions, we review his 1889 book The Psychic Life of Micro-Organisms: A Study in Experimental Psychology. This book details his observations of the behavior of microscopic organisms. We discuss each of the nine chapters and close with speculation of how his book may have helped develop his problem solving tests.