The California State Department of Public Health recently established a study file of all multiple births occurring in the state since 1905. The file contains birth and fetal death certificate information on nearly 78,000 sets of multiple births, from the influence of various demographic factors on twinning was analyzed. There is no indication that the annual frequency of twinning in California changed during the past 40 years. Considerable differences in respect to dizygotic twinning were observed among the racial groups. For every 1,000 confinements, dizygotic twins occurred with a frequency of 2 in Chinese and Japanese, 7 in whites and 11 in Negroes. Monozygotic twinning was practically unrelated to race, age of mother and parity. It occurred with a frequency of about 4-5/1,000 confinements. Both maternal age and parity appear to be strongly related to the occurrence of dizygotic twins. Among whites the maximum twinning rate occurred at mother''s age 35-39; among Negroes there was no evidence of a decline among older mothers. Parity and maternal age appear to operate independently. Negroes have much higher dizygotic twinning rates than whites even with these 2 factors held constant.