Abstract In Bolivia, Peru and Ecuador informal potato seed systems are much more important than formal systems. National seed projects recognize complementarities between the two systems and are seeking to link them. Arguments used to underpin linkages are assessed in the light of available evidence and accepted with some reservations. Project strategies have ranged from seed injection with minimum intervention to extensive reform ofthe informal system. All of the projects have met with some success. A number of general lessons are drawn for trapping the potential of informal seed systems.