We present findings from a field experiment on team diversity. Individuals working as canvassers in an elections-related experiment were randomly assigned a teammate, a manager and a set of households to visit. This created random variation in the degree of horizontal diversity, vertical (or hierarchical) diversity and external (or client) diversity within each team. We find that ethnic diversity among teammates decreases team performance, while diversity along the vertical dimension improves effort and performance. The performance effect of ethnic homogeneity within teams is more than twice the size of the effect of homogeneity along gender or age lines. Results from a survey of the canvassers suggest that horizontally homogeneous teams organized tasks in a more efficient way, while vertically homogeneous teams exerted lower effort.