The article is devoted to analysis of indicators of illegitimate birth rates, as well as consideration of the causes of the ongoing changes. The source base is parish register books of Barnaul churches of the XIXth century. The author has come to a conclusion that the basic trend in an illegitimate birth in Barnaul was an increase until the 1870s. From 1805 to 1877 the rates increased from 4.6 to 17.1 %, and then they reduced to the level of the mid-century. Indicators of illegitimate birth rate in Barnaul in second half of the XIXth century are comparable in general with the Siberian ones. The growing number of illegitimate birth was associated with improved quality of illegitimate children registration, although it can be said that the registration of illegitimate children was well established by the beginning of the XIXth century. The level of illegitimate births is affected by several subjective nature factors. Indicators depended on the socio-economic status of the population in the region, the number of migrants. The social composition of the population also affects the rates of extramarital births. This can be seen by comparing figures of illegitimate birth rate in various parishes of Barnaul and in the context of the class composition of the population.