This article draws from the conceptual framework of techno-bureaucratic and developmentalist doxas to understand the social practice of responsibilization in governance of non-industrial private forestry (NIPF) in the Southern Highlands of Tanzania. More specifically, attention is paid to responsibilization in the use of genetically improved tree seeds among small-scale tree growers. Data for the study were obtained using documentary review, participatory observation, focus group discussions and interviews with actors at district and village levels. Semi-structured interviews involved government and village officials, small-scale tree growers and other community members as well as NGO representatives. In addition, relevant policy, legislative and project documents and other grey literature were collected and reviewed for triangualation. Data were recorded, transcribed and analysed with a particular focus on emerging themes and sub-themes as well as patterns. The article shows that greater control is exercised by NGOs in the process of governance aimed primarily at modernization of private forestry generally and NIPF in particular. Existing power relations among actors are shaped and legitimized through the promotion of interventions that accord greater importance to professional and expert knowledge than to local knowledge. In effect, this has created dependence among small-scale tree growers and a greater reliance on expert technical knowledge, which is reflected in greater importance being given to training by outside experts at the expense of local initiatives. High cost of genetically improved tree seeds stands in the way of accomplishment of responsibilization in governance of NIPF and, more specifically, among small-scale tree growers.