An arrangement for restricting the inflow of formation water from an underground formation to a hydrocarbon producing well, where, between the underground formation and a production tubing (38) located in the well, there is disposed at least one flow chamber (3, 33) connected to the production tubing (38), the flow chamber (3, 33), preferably via a filter (2) in one portion, being open to inflow of formation fluid and in communication with the production tubing (38) via at least one opening (7, 32), and where the flow chamber (3, 33) is provided with at least one free-floating body (4, 34) with approximately the same density as the formation water, the at least one body (4, 34) being designed by means of the closing of at least one opening (32) or choking, to reduce the inflow of formation water to the production tubing (38).