For benthic fauna endemic to hydrothermal vents, larval dispersal in the plankton is required for maintenance of populations and colonization of new vents. Dispersal distances in the plankton are expected to be influenced by vertical positioning into horizontal currents, and larval survival in the plankton as well as encounter rates for settlement cues may be influenced by swimming speed. Here, we present the first quantitative measurements of swimming speeds of polychaete larvae collected near deep-sea hydrothermal vents. We focused on three polychaete larvae of different morphotypes, with two morphotypes identified genetically to Capitellidae and Spionidae. Mean swimming speeds and helical parameters of the deep-sea polychaete larvae (measured at 1 atm) were similar to values reported for shallow-water polychaete larvae, with mean helical 3D swimming speeds ranging from 0.8-1.4 mm.s(-1). To account for swimming that deviated from helical patterns, we developed a new method to reconstruct 3D swimming trajectories, using the 2D track and larval orientation. Speeds were generally faster for downward swimming, with mean vertical displacement speeds ranging from 0.6-0.8 mm.s(-1) downward as compared with 0.2-0.6 mm.s(-1) upward. Mean swimming speeds and helical parameters differed among the individuals. Our results are a first step towards constraining a behavioral component in models of larval dispersal between deep-sea hydrothermal vents.