Abstract: 488 and 211 blood samples were taken from 288 common carps (Cyprinus carpio L.) and 145 grass carps (Ctenopharyngodon idellus G. et V.) respectively during the period of 1963-1964. Blood indices, such as number of red and white blood cells (RBC and WBC) per unit volume, hemoglobin content (Hb), haematocrit (Ht), ery-throcyte sedimentation rate (ESR) and the differential blood count (DBC) were determined for each sample.This paper gives the counting results and morphological descriptions of red and white blood cells as well as the regularity of their numerical and morphological variations. The statistical data of certain indices, supplemented by the results of certain designed experiments, reveal the principal causes of their seasonal changes in relation to gonadal development or reproductive activities.Under normal rearing conditions, there is only slight difference in the RBC counts (or Hb content) and in DBC values between individuals of the same fish species. Therefore, they may be used as normal physiological indices for pondrearing common carp and grass carp.