Individual maize cultivars may react differently to fertilisation. Thus differential fertilisation levels can potentially have a marked influence on profitability of maize production and also on optimal cultivar selection. Fertilisation trials are not normally cultivardirected. The effect of fertilisation on a cultivar can thus not be directly determined. Adaptation regression lines of cultivars were used to determine economic optimum fertilisation levels for different maize cultivars at different locations. The conditions for such determinations are that the reaction of at least one cultivar on fertiliser must be known, and that the relationships between yields of different cultivars must be available. The variances between observed and predicted cultivar yields are relatively small. The proposed method should thus be satisfactory for the prediction of yields.