An EM algorithm procedure is presented for the maximum-likelihood estimation of mating system parameters of mixed mating system models for both angiosperms and gymnosperms. One advantage of the procedure is the ability to accommodate any number of alleles in the mature population and pollen pool. Estimates of the outcrossing rate ([Formula: see text]) derived from the model are bounded strictly within the natural biological range (i.e., 0 ≦ [Formula: see text] ≦1).