Deep sea exploration by manned submersibles has expanded the known depth limits of macroalgae end provided techniques with which to describe and quantify deep water macroalgal communities intact. The questions addressed in this investigation include: Are deep water macroalgal communities taxonomically distinct from shallow water communities? What are the biogeographic affinities of the deep water macroalgae? Deep water benthic algal surveys were conducted with the submersible MAKALI'I (Hawaii Undersea Research Laboratory) on Penguin Bank, Hawaii (21° N. lat. 157.5° W. long.) and Johnston Atoll (16° N. lat. 169° W. long.). Algal depth zonation was recorded by visual observation, color video recording, still photography, and voucher specimens collected at each depth with a suction device and manipulator arm. Drift algae were distinguished from algae which were actually growing attached to the rock or sand substratum. At Johnston Atoll, Dictyota from shallow reef environments was commonly observed at great depth 000 m ) as loose drift. The deep water macroalgal composition was distinct from that found in shallow water and the species diversity of deep water algae was low compared to a similar depth range at Penguin Bank. The algal species composition at Penguin Bank was composed of both typically shallow species and distinct deep water species. Macroalgaewere found at greater depths (250 m) on Johnston Atoll than on Penguin Bank. Crustose coralline algae we:re the deepest dwelling algae at both locations. The deep water macroalgae of Johnston Atoll have a greater biogeographic affinity with Marshall Islands flora than to Hawaiian deep water species, end are dissimilar in this regard to the observed distribution of hermatypic coral species in these regions.