I present a phenotypic model of the selection of self-fertilization that applies to any mode of selfpollination. A novel factor, seed discounting-the loss of outcrossing maternal fitness accompanying an increase in self-fertilization-is introduced, and pollen discounting is reformulated. The model incorporates the effects of five principal factors-the cost of meiosis, inbreeding depression, reproductive assurance, and seed and pollen discounting. The model indicates that either complete cross- or selffertilization is selected, depending on the values of the parameters, but a number of additional factors can lead to intermediate frequencies of self-fertilization. Specific conditions for the selection of each mode of chasmogamous self-pollination and the effects of some special circumstances are derived by substituting the appropriate values of seed and pollen discounting in the general model. Geitonogamy and facilitated selfing are primarily nonadaptive by-products of mechanisms for outcrossing. Delayed selfing is most advantageous, but competing and prior selfing may be selected when structural constraints prevent delayed selfing or the prepotency of outcrossed pollen reduces seed discounting. The model predicts environmental modification of the frequency of self-fertilization and can explain the observed associations between frequent self-fertilization and both the annual habit and poor conditions for cross-pollination.