For the first time population immunity to virus hepatitis A has been studied during three different phases of prolonged morbidity cycles of this infection, and the results of this study have been compared with the data on morbidity in different age groups. Pronounced variability of the immunological structure of the population in different age groups, found to be related to the dynamics of hepatitis A morbidity, has been established. Fluctuations in immunity level are most pronounced among children aged 1-6 and 7-14 years, having the least proportion of seropositive persons. A new epidemic cycle is started among these groups of the population, and at the first stage this cycle is manifested by an increase in the intensity of the latently developing epidemic process. This is followed by the activation of registered morbidity among the whole of the population. Seroepidemiological study may be used both for prognostication purposes and in the system of surveillance on this infection.