Twinning rates for the years 1975-79 in Catalonia (Spain) are presented. Crude rates are very low: 7.62 per 1,000 maternities, the DZTR and the MZTR being 3.74 and 3.88 respectively. Standardized rates remain very low. Sex ratio among twin couples is also very low (0.49 male vs 0.51 female births). A multiple linear stepwise regression on the twinning rates shows MZ rates to be influenced by birth order and father's age, and the DZ rates by mother's age and birth order.