The haemogram (blood cells concentration, number and proportions of haemocyte morphotypes) of the common periwinkle, Littorina littorea, was studied in terms of its individual variations and seasonal dynamics. No granular blood cells were found. Three morphotypes of hyalinocytes were observed in the haemolymph of L. littorea and are described at both light and electron microscopy levels: juvenile round cells, intermediate cells and large mature haemocytes. The nucleoplasmic ratio, nucleus shape and location, number and size of glycogen deposits were the main characteristics used to discriminate between the haemocyte morphotypes. The enzyme cytochemistry and lectin binding studies did not identify additional haemocyte morphotypes. The experiments on BrdU incorporation showed that haemocyte population renewal occurred due to the proliferation of only one cell type, namely blast cells, which were morphologically similar to juvenile round cells. The three-month dynamics of the periwinkle haemogram and haemoblast proliferative activity are described. The blood cell concentration reached maximum values twice per summer season—in June and August, with the haemocyte proliferation activity being the highest in June. The percentage of mature haemocytes increased during the season, simultaneously intermediate cells demonstrated a decrease in number by the end of summer, with the population of juvenile cells being nearby constant. The method of haemogram analysis is positioned as a powerful technique having good potential in eco-toxicological, parasitological and immunobiological studies.