The Mexican Maize Institute, in Saltillo, Northern Mexico, has developed maize populations with polyembryonic traits. Polyembryony (PE) is interesting for at least two reasons: two or more embryos per seed can produce more oil and embryo protein, as well as savings in the number of seeds necessary to reach a certain population per unit area. The founding population (established in 1973) had 1.5% of PE, identified as "twin plants" by seed. Starting in 1975 different recurrent selection schemes have been applied in order to increase the PE frequency. By 1991 the selection response placed PE at 47%; then the base population was divided into two groups: brachytic and normal height. By 1996 these two populations showed a PE frequency above 60%. For the last 5 years, reproduction management of polyembryonic groups has been made through parental crosses with a mixture of pollen. Between 200 and 250 halfsib families (SHS) were selected in a given cycle in the field in each group; each SHS is represented by 50 seeds, which were individually planted and kept under greenhouse conditions. The families to be transplanted were selected from these according to their PE proportion and at least 75% germination. PE increase seems to limit seed germination to an average lower than 90% under greenhouse conditions, and lower than 60% under direct planting.