Since the advent of deep-sea microbiology in the 1950's, microbiologists have justifiably recognized Adrien Certes as a pioneer due to his early studies on culturing deep-sea bacteria, and investigating the effects of pressure on microorganisms. However, Certes was actually first and foremost a protistologist. He was but a 'part-time' scientist, without advanced training in biology, and only began publishing in middle age. His 'day job' was a high-level civil servant, an Inspecteur des Finances. Nonetheless, he authored over 50 works on a remarkably wide range of free-living and commensal protists as well as microscopical methods. He interacted with major figures of biology in France of his time such as the protistologist Édouard-Gérard Balbiani, the microbiologist Louis Pasteur, and the zoologist Alphonse Milne Edwards, although exactly how his connections were forged unfortunately remains obscure. Certes, acknowledged by marine microbiologists, is largely overlooked in accounts of protistology, perhaps because his work was dispersed over a variety of topics and taxa. Also, as a part-time researcher, he left no academic children behind. Here the life and scientific work of Certes are reviewed and it is shown that he was a devoted protistologist.