This paper describes a technique for using plot surveys to measure individual informants' ethnobotanical knowledge of forests, as applied to the Dusun community of Merimbun in Brunei. Two knowledgeable but non-literate Dusun informants enumerated marked plots of both recent and old secondary growth mixed dipterocarp forest near the village. They were able to provide names (other than life-forms or the most general basic and intermediate categories) for 86-97% of species growing in the plots. Between 152 and 170 plant names were elicited by the surveys. In all cases, about 88% of the names were at the basic naming level and 12% below. The surveys reveal the breadth of biodiversity knowledge of particular types of forest and highlight differences in the knowledge of individual informants and the ways in which that knowledge is organized. The plot-survey technique provides a way of measuring the comprehensiveness of local knowledge of plants with reference to all plant types found within circumscribed plots in locally recognized biotopes, and may be useful as a rapid means of assessing local ecological diversity.