The mortality rate (stillbirths and infant deaths) from anencephalus from 1950-1969 in 36 cities of over 50,000 population in Canada showed a negative association (r = -.39) with the concentration of magnesium in water sampled at domestic taps. The mortality rates showed negative associations with mean income and longitude, and a multiple regression model using the three factors showed significant effects of each and accounted for 69% of the intercity variation in rates. There were no significant associations seen with water calcium concentration or total hardness. Income, magnesium and longitude were also negatively associated with mortality rates from spina bifida, hydrocephalus, other congenital abnormalities, and total stillbirth and infant death rates, but the association with magnesium was significant only for total stillbirths. The negative association of anencephalus mortality and magnesium levels was also seen in a sample of 14 smaller towns in Ontario.