Technical Abstract: Maximizing intake of lactating dairy cows is crucial in maintaining high levels of milk production, especially when grazing. Intake can be strongly affected by the amount and type of forage offered to the animals. Pastures containing multiple plant species have been shown to have greater plant productivity than pasture planted with a single plant species. However, it has not been determined whether this increase in plant productivity translates into increased animal productivity. Therefore, a study was designed to evaluate the effects of varying levels of plant species diversity on intake, grazing behavior, and productivity of lactating dairy cows. Sixteen lactating dairy cows were grazed in four groups on one of four pasture treatments. Pasture treatments were: a two-species forage mix, a three-species mix, a six-species mix, or a nine-species mix. Cows received supplementation and were grazed for 21 days on each pasture treatment and then switched treatments so that all cows were on all pasture treatments by the end of the trial. The results indicate that the level of plant species diversity on pastures did not have a major impact on intake or productivity of grazing dairy cows. However, forage production and carrying capacity of the pastures were greater on the more complex mixtures.