French biologist best remembered for his work on the subterranean coleopterans of the family Anisotomidae. His exploration of the caves of the Pyrenees and Carpathian mountains yielded many species of these small, shiny, round fungus beetles that were hitherto unknown. His fieldwork in Africa also resulted in the identification of various previously unstudied subterranean insects. Such efforts did much to stimulate speleological studies.

Jeannel also made significant contributions to biogeography, the study of the geographical distribution of living organisms, particularly in France. His books La genèse des faunes terrestres (1942; “The Origins of Terrestrial Fauna”) and Faune cavernicole de la France (1940; “The Fauna of the Caves of France”) remain standard references in this field of study. Jeannel's intimate knowledge of the fauna of France and its neighbouring regions enabled him to develop an outstanding vivarium (an enclosure for raising and observing animals indoors) and a diverse entomological collection at the Museum of Natural History in Paris, of which he served as director from 1945 to 1951.