Abstract
The freshwater diatoms of the French Guiana have received little attention so far. A new diatom genus, Lacuneolimna, based on the species Eolimna zalokariae, is described from a small rainforest stream. Numerous ultrastructural features differ from the genus Eolimna, notably the pluriseriate striae lying between relief-like elevated virgae and depressed externally. In addition, another species new to science, L. novagallia, belonging to this new genus, was discovered in the same place. Detailed light and scanning electron microscope observations are used to characterize the morphology and the ultrastructure of these two new taxa.