ABSTRACT: “Codex canadiensis” consists of 79 leaves with 180 illustrations of plants, birds, mammals, fishes, and a few fabulous animals. This manuscript arguably is the most obscure and enigmatic surviving document pertaining to the early natural history of French Canada. It was lost until 1930, when Baron Marc de Villiers first published a facsimile. Two inferior editions later appeared in Canada. The codex was acquired about 1949 by Oklahoma oil baron Thomas Gilcrease and then deposited in the Gilcrease Museum, Tulsa, Oklahoma. Under the direction of one of us (Gagnon), French-Canadian scholars have established the codex's author was Father Louis Nicolas (1634–c. 1678), a Jesuit priest who laboured among tribes along the St Lawrence River and the Great Lakes during 1664–1675. This rejects the previous attribution to Charles Bécard (correctly Bécart), Sieur de Granville. The codex likely was completed in part, if not entirely, after Nicolas' return to France in 1675, and it is closely related to a much larger undated work by Nicolas, “Histoire naturelle des Indes Occidentales”. “Codex canadiensis” is among the most valuable extant manuscripts illustrating the natural history of North America as explored by early European naturalists.