Hematological parameters are used as an essential tool to assess the health status of fish. This study aims to provide a background on hematology in Lake Tonga's Cyprinus carpio (Linnaeus, 1758) (Pisces Cyprinidae) and to demonstrate the impact of seasonal variations, sex and size on hematology. The study was conducted throughout 2018 and involved 120 individuals sampled monthly and randomly. The specimens were weighed and measured. Blood samples were collected to determine hematocrit (Ht), hemoglobin (Hb), red (GR) and white (GB) blood cell count (Lym, Mon, Gra) and erythrocyte constants: mean blood cell volume (VGM), mean blood cell content (TGM), mean corpuscular hemoglobin concentration (CCMH). The results revealed that the majority of the parameters studied did not show significant differences in size classes, and the statistical comparison between the two sexes revealed significant differences in the values of GR, GB, Mon, Mon, Gra, Ht, Hb, TGM and CCMH. On the other hand, all the parameters studied varied significantly over the seasons.