More than 130 specimens representing various growth stages of Rehbachiella kinnekullensis Müller, 1983, have permitted a detailed description of its ontogeny. It begins with a nauplius already able to swim and feed actively. The 30th stage is about 1.7 mm long, but still immature. Because the type specimens belong to earlier instars, the original diagnosis of Muller (1983) is emended. Details of the limb apparatus of late instars suggest that the animals were able to filter-feed by this stage, possibly while swimming close to the bottom. Two larval series are distinguished by size and morphology in their early stages, but their structural differences become almost balanced subsequently. This is interpreted as intraspecific differentiation rather than as existence of two species. The entire postnaupliar feeding apparatus of Branchiopoda, which is basically adapted to filtration, is recongnized here as an apomorphic character of this group. Branchiopoda comprise the two monophyletic units Anostraca and Phyllopoda (Calmanostraca, with Notostraca and Kazacharthra, and Onychura). Rehbachiella shares all major aspects of the branchiopod apparatus, which led to identify it as an ancestral marine branchiopod. Moreover, there are indications that Rehbachiella is a representative of the anostracan lineage, i.e. a representative of the stem-group of Sarsostraca, which include the Devonian Lipostraca and the extant Euanostraca. The long larval sequence of Rehbachiella and selective external features, including the locomotory and feediang apparatus, are evaluated for their bearing upon the phylogeny of Branchiopoda and Curstacea in general. This study on Rehbachiella supports the monophyly of the crown-group Crustacea (sensu Walossek & Muller 1990). It also has revealed that only the first maxilla was morphologically and functionally included into the crustacean head, while subsequent limbs were addted to the head in a stepwise manner and became modified separately within the different crustacean lineages, which is of great relevance when evaluating the relationships between these.