For the first time Amphipoda have been discovered living in Bryozoa. A new genus and species of the amphipod family Chevaliidae, Bryoconversor tutus gen. et sp. nov. is described from New Zealand at 530–1500 m depth. The species lives in an inquiline relationship with the cheilostome bryozoan Onchoporoides moseleyi (Calwelliidae), inhabiting an abfrontal basal coelom of the bryozoan beneath the membranous ectocyst (cuticularized epithelium) that conceals and protects the amphipods. The colony is strengthened along all edges by a unique intracoelomic rod of calcium carbonate that is formed within the marginal kenozooids of the colony. The potential benefits and costs to the bryozoan are discussed.