To date, little of the ecological research on biological diversity and ecosystem functioning has been carried out in agricultural systems, despite the fact that agriculture is a major contributor to loss of native habitats and species. However, agricultural research has demonstrated that polycultures of multiple crop species can have higher total production than monocultures of single crop species. In this article, the authors describe how they designed a laboratory for their students for the purpose of exploring the relationship between biological diversity and ecosystem functioning in an agricultural system. In this lab, students record data from an experiment set up several weeks earlier to answer the question: How is plant biomass related to number of plant species? Later, students use subsets of the data to test hypotheses about mechanisms that could cause primary production to change with plant species richness. The authors present results from a college-level introductory biology course. The amount of preparation and number of treatments probably preclude its use at the high school level, although data are available from the authors to supplement smaller experimental designs. (Contains 3 tables and 5 figures.)