The responses of rates of net photosynthesis (F^) to photosynthetic photon fluence rate (i), leaf temperature and CO2 were studied inhalf-sib families of greenhouse and field grown black locust (Robinia pseudoacacia L.) seedlings. The response of to 1 was similar in both greenhouse and field grown seedlings, except the quantum yields were lower in the field grown seedlings. The saturáting / was as low as 600 pmol m'^ s'’ in some seedlings but did not saturate at 1900 púiol m'^ s'^ in others. The optimum temperature for in field grown seedlings was shghtly lower than the greenhouse grown seedlings. increased rapidly with increased CO2 up to 350 pmol mol'^ but beyond SOO pmol mol'* the increase was gradual in some families. On average, stomata represented only 23 % of the limitation to Pii with no differences between families observed. Family x I, family x temperature and family X CO2 were not significant for Pj^.