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From birth certificates for Nuevo Leon for the years 1977 and 1978, twinning rates per 1000 maternities of 2.7 MZ and 4.3 DZ and a 0.63 MZ/DZ ratio have been found. The total and MZ twinning rates are intermediate between those for Spaniards and Indians. Both MZ and DZ rates are positively related with mother s age, from 16 to 35, and father’s age, from 16 to 45, but are negatively related after those ages. When both parents’ ages were considered simultaneously, it was found that twinning rates are more related to mother’s than father’s age, especially the DZ rate. There were positive relations between MZ and DZ rates and birth order; and when birth order was considered simultaneously with mother’s age, it was found that the rates are more related to birth order. There are different monthly twinning rates and MZ/DZ ratios, but these differences are not significant when the months were grouped by season of the year. There are no significant differences in the twinning rates and ratios between zones of the State or between rural and urban populations.