Seventeen species of cirratulid polychaetes, 13 new to science, are reported from continental shelf and slope depths of the western North Atlantic Ocean. The samples were collected as part of deep-water offshore reconnaissance surveys from New England to the Carolinas and long-term monitoring programs in Boston Harbor, Massachusetts Bay, and Georges Bank off Massachusetts. The new species are included in the genera Caulleriella (C. cryptica n. sp.), Chaetocirratulus (C. hessleri n. sp., C. sandersi n. sp., and C. tomaculus n. sp.), and Chaetozone (C. adunca n. sp., C. artaspinosa n. sp., C. brychiata n. sp., C. castouria n. sp., C. donerae n. sp., C. lophia n. sp., C. novagracilis, n. sp., C. paucispinosa n. sp. and C. profunda n. sp. In addition, Chaetozone gayheadia Hartman, 1965 is redescribed based on type-material and additional collections and transferred to the genus Chaetocirratulus. Chaetozone benthaliana McIntosh, 1985 is designated a species inquirendum because it was described from a posterior fragment that cannot be confirmed as belonging to any known cirratulid genus or species. Updated descriptions and new records are provided for Chaetozone anasima, C. diodonta, and C. hystricosa all originally described by Doner & Blake (2006).

Seven new species of Cirratulidae are described from deep waters off the east coast of Australia. Samples were collected as part of the RV Investigator voyage IN2017_V03 in May/June 2017 using a Brenke sledge and 0.25 m2 box core. Sample depths reported in the present study were from the lower continental slope of about 2100 m to abyssal depths up to 4170 m. These collections provide the first cirratulid polychaetes to be described from deep water off Australia. The new species of Cirratulidae are in the genera Aphelochaeta (2), Chaetocirratulus (2), Chaetozone (2), and Kirkegaardia (1). Each of the new species is compared and contrasted with their known congeners. The bitentaculate Cirratulidae known from abyssal depths of 3000 m and greater are reviewed and discussed.

Seven new species of Scalibregmatidae are described from deep waters off the east coast of Australia. Samples were collected as part of the RV Investigator voyage IN2017_V03 in May/June 2017 using a Brenke sledge and 0.25 m2 box core. Sample depths reported in the present study were from the lower continental slope of about 2450 m to abyssal depths up to 4280 m. These collections provide the first scalibregmatid polychaetes to be described from deep water off Australia. The new species of Scalibregmatidae are in the genera Asclerocheilus (1), Axiokebuita (1), Oligobregma (4), and Pseudoscalibregma (1). Each of the new species is compared and contrasted with their known congeners. Asclerocheilus and Oligobregma are the largest genera of the family in terms of numbers of species; the known species of each of these genera are tabulated and compared. The Scalibregmatidae known from abyssal depths of about 3000 m and greater are reviewed and discussed.