The article identifies and defines three extension domains among urban farmers in the informal settlements of Atteridgeville. These consist of home gardeners, community-project gardeners, and users of open urban spaces. The extension domains are defined in terms of type of space used for farming, choice of crops and related production practices, access to resources, and the socio-economic characteristics and important agricultural needs of farmers. This information was obtained by means of a survey (n=120) involving face-to-face interviews and a structured interview schedule. The survey was complemented by semi-structured interviews and observations of agricultural activities. The paper concludes by calling for closer involvement of government and public extension in urban agriculture in the study area, because urban farming mainly benefits the poor. Specific constraints, which extension services may help to address, are identified for each of the three extension domains.