This case study describes the process of participation and incidence of a civil society organisation, Foundation for Urban Development (FUDEU) and other social actors in the elaboration of a new water law in Costa Rica, based on their own local experiences in the 'Grande de Tarcoles River Basin Commission'. In 2000, FUDEU's research suggested that the existing framework prevented the Commission from assuming necessary competences and legal responsibilities to be a real river basin agency. Hence FUDEU decided to promote the elaboration of a new water law in Costa Rica by creating the 'Technical Group of Water' (GTA) with other social organisations, governmental institutions and international organisms. The GTA developed the widest process of dialogue and discussion ever made around a law in Costa Rica.