Adenocystis and Utriculidium share many morphological characters. They both possess a hollow, bladder-like thallus, made of a solid cortex and a fluid-filled medulla penetrated by a network of longitudinal filaments, a trichothallic apical meristem responsible for their multiaxial structure, and cryptostomata. There are two somatic phases in their life history: a haplostichous macrothallus and a polystichous microthallus. However, Adenocystis has unilocular sporangia associated with unicellular paraphyses resembling those of the Laminariales, whereas Utriculidium has plurilocular sporangia resembling those of the Scytosiphonales. Adenocystis has been classified either in the Laminariales or in different families of the Ectocarpales sensu lato, or suggested to be close to the Scytothamnales. Utriculidium has also been classified in different groups within the Ectocarpales sensu lato, depending on the authors. The two genera were either considered as closely related and even to be phases of the same complex life history, or placed in separate orders. The life histories obtained in culture have suggested that Adenocystis and Utriculidium are independent taxa. The taxonomic positions of the two genera Adenocystis and Utriculidium, which are currently unresolved, were explored in the present study, using phylogenetic analysis of combined partial LSU and SSU rDNA sequences. A data set of 20 species included Chorda, Chordaria, Dictyosiphon, Laminaria, Punctaria, Scytosiphon and Scytothamnus, the type genera of the families in which Adenocystis and/or Utriculidium were sometimes previously placed. Adenocystis and Utriculidium grouped together in all analyses within Ectocarpales sensu lato but failed to group with the ectocarpalean genera Chordaria, Dictyosiphon, Punctaria or Scytosiphon, as previously hypothesized. In fact they did not show any clear affinity with any other ectocarpalean taxa. On the basis of this result, and morphological data, we propose a new family, Adenocystaceae, to include these two genera. It is hypothesized that their current life cycles derive from an ancestral isomorphic life cycle.