The study had as objective to determine the critical and lethal water contents of assai seeds. For such, mature fruits of three ecotypes of the species, belonging to the germplasm bank of palm trees of the Agronomic Institute of Campinas (Ubatuba, State of São Paulo) were harvested and transported, in impermeable packing, to Faculdade de Ciências Agronômicas/UNESP (Botucatu, State of São Paulo), where they were shelled. The seeds were submitted to the dehydration in drying chamber, reaching decreasing water contents starting from the not dehydrated control (40,4 to 41,0% of initial water content). The effects of the dehydration were evaluated by means of the standard test of germination; percentage and speed of protrusion of the embryo, emission of the germinative button and plumule; seedlings length; determination of water contents; and electric conductivity after soaking the seeds for one day. It was found, for the studied species, a critical water content band varying from 34,2 to 36,4%, while the lethal water content was detected between 17,4 and 18,9%. Although those contents have been similar for the three ecotypes in study, there were differences in relation to the germination and vigor, indicating that those last ones are under genetic control. The seed water content was always positive and highly correlated with characteristics that evaluate germination and vigor. The electric conductivity of the soaked water was efficient to detect low viability of seeds, always showing negative correlations when pared with germination and water content. However, caution is recommended in its use, because the critical bands, above which the germination is seriously harmed, varied with the ecotypes in study.