SummaryAn attempt has been made to outline the genetical demography of a geographically relatively isolated north Swedish population during 1900–50. The region, which is divided into three parishes, P, J and M, was characterized by high birth‐rates and low in‐migration rates. The main results were as follows:(1) The mean inbreeding coefficient was estimated at 0·002 for the total population and for the parishes P, J and M at 0·0008, 0·0024 and 0·0058 respectively.(2) The mean effective population size of the parishes P, J and M was estimated at about 650, 300 and 150 for the period 1900–50.(3) The analysis indicated that the population of the total area should not be regarded as one breeding unit but rather a minimum of 3.(4) The distribution of two well‐defined simple recessive morbid conditions was studied. Genetical spastic oligophrenia, an entity discovered in this population, occurred with an average frequency of 2·6 per 1000 births. The incidence in the most isolated parish, M, was significantly high as compared with P + J. Recessive deaf mutism occurred with a frequency of 1·3 per 1000, which is close to three times the average for Sweden. This difference, and the occurrence of a specific genetical defect in this population, may be interpreted as mainly due to random ge