Ladino clover ( Trifolium repens L.) was subjected to two 2-h exposures, one week apart, of carbon-filtered air, 30 pphm ozone or 60 pphm ozone, in the facilities of the Southeastern Plant Environmental Laboratories. Plants were exposed at the following times: 14 + 21, 28 + 35, 42 + 49 or 56 + 63 days after seeding. Plants at each exposure age were harvested over a four-week period starting one week after the last exposure. Ozone reduced the growth of tops and roots. The magnitude of the effect varied with ozone concentration and plant exposure age. Both 30 and 60 pphm ozone reduced nodulation for the first three exposure ages. Nitrogenase activity per plant and per nodule was not significantly modified when compared with the appropriate controls. Total nitrogen was directly correlated with biomass.