The purpose of this guide is threefold. In the first place, it serves as an aid for the identification of the majority of native and naturalized vascular plants in central French Guiana (Fig. 1). In the second place, it is a guide for recognizing plant families and genera within lowland forests throughout most of northeastern South America because many of the same taxa of plants found in this flora also occur throughout the lowland Guayana floristic province of northeastern South America (Mori, 1991). Finally, it provides a glossary of the terminology used for describing the plants of the lowland forests of the Neotropics. This first volume includes the pteridophytes, the single gymnosperm, and the monocotyledons. The second volume will treat the dicotyledons.