The paper analyzes seasonal fluctuations in mortality among the parish population on the basis of parish registers of the Pokrovsky parish in Barnaul in the last third of the XIX century. The study results show that infant mortality in Barnaul in 1877-1897 was still strongly dependable upon the time of year. Summer and winter peaks suggest a continued role of exogenous causes of death. Infant mortality rates in summer were three times higher than the annual average. The mortality peak is mainly registered in June. Starting from the age of 10-14, the seasonal factor passes from the picture, and deaths are distributed more evenly over the seasons. The analysis of seasonal fluctuations in mortality by cause of death showed that in children, summer peaks are associated with infectious diseases, while winter peaks are due to cold-related diseases. Seasonal fluctuations in mortality by social class are not fundamentally different. It can be assumed that the demographic transition was still in its early stages and yet to be developed even in the upper classes, which had always been at the forefront of demographic modernization. The obtained results and conclusions about seasonal fluctuations in infant and child mortality are consistent with pre-revolutionary and modern research. The existing differences in seasonal fluctuations in infant mortality exemplified by different cities and settlements in Russia with peaks either in July or August, may indicate a wide climatic diversity and socio-economic differences between regions of the country, as well as differences in calculation methods.