The subaerial green algal order Trentepohliales has its centre of abundance and diversity in the tropics. However, very few detailed investigations on the taxonomy of this group are available for tropical regions. Collections made in the course of a fieldtrip to French Guiana in June 2006 have revealed a very high diversity of the Trentepohliales in the region. Twenty-eight taxa were recorded; many of them were associated with humid and shaded rainforest habitats. Two undescribed species (Trentepohlia chapmanii and T. infestans) were found and five taxa represented new records for the Americas (Phycopeltis irregularis, Printzina bosseae, Trentepohlia cucullata, T. diffracta var. colorata, T. dusenii). On the basis of these results and literature data, the trentepohliacean flora of French Guiana amounts to twenty-nine taxa. Comparison with other tropical areas shows that this is a particularly high number; French Guiana can be therefore considered a biodiversity hotspot for the Trentepohliales. The combination of a highly humid and rainy climate with a high richness of habitats provides very suitable conditions for the development of these algae. Combined with evidence from other studies, the results indicate that tropical rainforests represent a major repository of unexplored microalgal diversity and that extensive investigations on the microalgal flora of these environments are urgently needed.