Choanoflagellates are the closest living relatives of animals and, as such, provide insights into the origin of key features of animal biology.

Interactions with bacteria are essential for choanoflagellate nutrition and life history. Bacteria are the primary food source for choanoflagellates.

Here, we report the isolation and characterization of a new choanoflagellate species, B. monosierra, that forms large colonies containing bacteria. To our knowledge, this is the first report of such an interaction between choanoflagellates and bacteria.

Finally, the close phylogenetic relationship between choanoflagellate and animals suggests that the interactions among B. monosierra and its bacterial residents have the potential to illuminate the ancestry of and mechanisms underlying associations between animals and bacteria.