To assess the effects of O-3 on growth, net photosynthesis and yield of two African varieties of cowpea (Vigna unguiculata L.), Blackeye and Asontem were exposed as potted plants to air that was either filtered to remove O-3 (FA), non-filtered air (NF), non-filtered with added O-3 of approximately 50 nL L-1 (ppb) from 11:00 to 16:00 (NF + O-3) for 88 days in open-top chambers. The mean O-3 concentration (11:00-16:00) during the exposure period had a range from 16 ppb in the FA treatment to 118 ppb in the NF + O-3 treatment. Net photosynthetic rate and leaf area per plant were significantly reduced by exposure to O-3, reducing the growth of both varieties. Exposure to O-3 significantly reduced the 100-seed weight and number of seeds per pod. As a result, cowpea yield was significantly reduced by long-term exposure to O-3, with no difference in sensitivity between the varieties. (C) 2014 Elsevier Ltd. All rights reserved.