Biomass is a poorly documented parameter of the hydrothermal ecosystem, partly due to the difficulties in estimating it. Because the hydrothermal alvinellid polychaete worms mostly live in tubes, and on very irregular surfaces difficult to sample, an original biomass estimation method based on video analysis was proposed and tested on populations of the East Pacific Rise at 13-degrees-N. The area covered by the 3 main alvinellid species was accurately estimated from geometrical calculations in oblique-angle conditions, requiring only 3 object measurements on the monitor screen. The total density of alvinellids was estimated on video by modelling the pattern of worm movements in and out of their tubes. The Richards function was found appropriate to model the resulting asymptotic curves. Biomass was calculated from mean alvinellid weights for populations from 3 white smokers and 1 black smoker. Values ranged from 10.7 to 70.1 g ash-free dry weight m-2, which is a much lower estimate than in previous studies, perhaps because of differences in sample size, sampling methods and the heterogeneity of studied assemblages. It was possible to discriminate Alvinella spp. from Paralvinella grasslei on the video because of differences in behaviour, allowing separate biomass calculations. Spatial variability of the alvinellid biomass was assessed f or one chimney. Biomass was maximal at the top and the base, and minimal in the middle. Coverage of the chimney by the alvinellid tubes was spatially heterogeneous, and the genera displayed different distribution patterns. P grasslei seems to move a lot and settle on chimneys in an opportunistic way, while Alvinella spp., more dependent on the environmental conditions, settle at the base and gradually move upwards as the chimney grows.