Because of how heavy water is produced and used, it has a unique business history and structure. In the late 1940s and 1950s, heavy-water reactors offered the dream of nearly inexhaustible quantities of nuclear power generation. This was because they could operate using natural uranium and also produce plutonium that could be used as fuel in breeder reactors (and also as fissile material for nuclear weapons). Several of the processes and materials used in synthetic fertilizer production could also be used, in somewhat modified form, in the production of heavy water. Yet the production costs for heavy water are exceedingly high, requiring copious amounts of electricity and the infrastructure of an advanced chemical industry. This article suggests that in several countries the fertilizer and heavy-water industries had a close relationship. The governments in those countries, seeking to increase the national trade in fertilizers and to develop domestic nuclear programs, supported both industries. The case of the Spanish Nuclear Board and Energía e Industrias Aragonesas in Sabiñanigo is instructive in this regard.