This paper is about rights to water. It follows a conflict over use of groundwater in Mexico from the middle of the 1960s. It illustrates how previously defined rights concerning access to water resources can be eroded. Two main mechanisms seem to be involved. First, the character of the resource - groundwater - makes it difficult both to distribute rights and to define when they are violated. Second, unequal power to express ones interests and to influence the process of the de facto shifting of rights is of great importance. While the Valley of Ixtlahuaca prior to the 1960s was blessed with ample access to water, extraction to supply Mexico City, which has been taking place since 1966, lowered the water table rapidly. After a few years traditional access to groundwater by locals was made impossible and the functioning of the local ecosystems was disrupted. It is shown that the redistribution of access to water was not in accordance with the existing legal rules concerning water rights. Owing to continuing water shortage, a pricing system is proposed forcing previously right holders to pay for the water in the future.