CIMMYT's methods for on-farm research (OFR) are now being implemented by many national programs, helping them to identify improved and appropriate technologies for target groups of farmers. These experiences have contributed to a growing awareness that policy-induced constraints can limit gains associated with potential or actual technological change. CIMMYT's Economics Program has recognized that efforts to analyze this policy context and effectively communicate information derived from the analysis to policymakers could improve the basis for formulating or implementing policy. This area of research is called farm-based policy analysis (FPA). The case of market imperfections in fertilizer provision in Haiti discussed in this working paper illustrates the close links between OFR and FPA. However, although traditional OFR assumes that socioeconomic circumstances--including the policy environment--are a given, FPA sees policy as a variable, and builds a case for modifying policy constraints by applying microeconomic tools to farm-level data obtained through OFR programs. As this study demonstrates, that approach helped encourage important changes in fertilizer policy in Les Cayes, Haiti, which suggests that the market imperfections identified through the analysis were due at least in part to a lack of appropriate technical information among the relevant decision makers. It is apparent that FPA analysis using data generated from OFR programs has much potential to help correct such deficiencies and make farmers' policy environment more conducive to technological change.21 pages