The role of domestication in conserving indigenous plant diversity on farmlands was studied in West Usambara Mountains, Tanzania by structured, semi-structured and unstructured interviews; participant observation; literature review and botanical identification in March 2006. There was high diversity of indigenous plant species on farmlands (73%) compared with exotics (27%). For the number of plants per species, a small proportion of indigenous plants (14%) were recorded compared with exotics (86%). Although the number of domesticated indigenous plant species has declined by 8% between 1998 and 2006, still 96% of the respondents practise domestication and they have domesticated at least one indigenous plant species each. Gender equity in domestication is lacking given that male-headed households had domesticated significantly more plant species on farms than female-headed households. Sustainability in domestication is questionable because of increasing number of exotic plants of very few species on farmlands, the phenomenon that encourages monoculture, thus threatening conservation of species diversity in inhabited areas. Suggestions for promoting domestication as an important indigenous method for conserving biodiversity are given, including the need for more number of studies to explore the biodiversity of other organisms associated with the domesticated indigenous plants.