Systematic sequencing of environmental SSU rDNA genes amplified from different marine ecosystems has uncovered novel eukaryotic lineages, in particular within the alveolate and stramenopile radiations. The ecological and geographic distribution of 2 novel alveolate lineages (called Group I and II in previous papers) is inferred from the analysis of 62 different environmental clone libraries from freshwater and marine habitats. These 2 lineages have been, up to now, retrieved exclusively from marine ecosystems, including oceanic and coastal waters, sediments, hydrothermal vents, and permanent anoxic deep waters and usually represent the most abundant eukaryotic lineages in environmental genetic libraries. While Group I is only composed of environmental sequences (118 clones), Group II contains, besides environmental sequences (158 clones), sequences from described genera (8) (Hematodinium and Amoebophrya) that belong to the Syndiniales, an atypical order of dinoflagellates exclusively composed of marine parasites. This suggests that Group II could correspond to Syndiniales, although this should be confirmed in the future by examining the morphology of cells from Group II. Group II appears to be abundant in coastal and oceanic ecosystems, whereas permanent anoxic waters and hydrothermal ecosystems are usually dominated by Group I. Based upon the similarity of partial sequences, we organized these 2 groups into clusters, The diversity of Group II (16 clusters) is wider than that of Group 1 (5 clusters). Two clusters from Group I have a widespread distribution and are found in all explored marine habitats. In contrast, all other clusters seem to be limited to specific marine habitats. For example, some clusters belonging to Group I and Group II are only detected in extreme environments (anoxic and hydrothermal vents), whereas many clusters from Group II have only been retrieved from coastal waters. We determined near-complete SSU rRNA gene sequences for 26 environmental clones, selected in order to obtain at least one complete sequence per cluster. Phylogenetic analyses (maximum likelihood, neighbor joining, maximum parsimony, and Bayesian reconstruction) based upon complete sequences all concurred to place both Group I and II as sister lineages of dinoflagellates. This result contradicts several published studies, which placed both groups within dinoflagellates.