To understand the tube growth process of Riftia pachyptila, morphological aspects of worms and their tubes were studied. In parallel, tube secretion experiments were performed on live animals, in pressure aquaria. Dry weights of the secretions, along with their chitin content (a major component of the tube) were used to quantify tube production. Our results show a variation of the gross morphology of the plume and the trunk of R. pachyptila during its growth and indicate that vestimentum length and tube diameter could be useful indices of individual and tube sizes of R. pachyptila. The presence of clumps of freshly secreted tube material at the base of the exoskeleton as well as the new observation of bifid tubes allow us to propose a model of tube growth at both ends. In this model the tube growth would exhibit a moulting-like step. Bifid shapes may help in space displacement, and the modification of the positioning in height relative to a vent could be used by an individual to modify its access to vent fluid. Tube growth at the apical end facing the plume has been quantified as a minimal estimate of the rate of tube production. Experiments performed on repressurized worms indicate that 1 mm(2) of worm vestimentum area may secrete more than 2 mu g of dry weight tube material per day, leading to a minimum tube growth rate of 14 cm yr(-1). When compared to other marine ecosystems, it is obvious that chitin production per unit of area of the the vent communities, based on the R. pachyptila alone, are the highest recorded and similar values were only recorded in polluted freshwater environments.