Hematological parameters were compared in O. niloticus weighing between 50-150 g and 150-250 g, for comparison referents, carried out at the Fishing Station, Universidad de Caldas, Colombia. A completely randomized design was used with sub-sampling; the blood samples were placed in test tubes with EDTA for analysis in the laboratory. SPSS software, the Kolmogorow-Smirnov test, T test, Levene and the Kruskal Wallis tests were applied for correlations between hematologic parameters evaluated. When correlating the two weights ranges in Oreochromis niloticus no meaningful differences were found in the hematologic values. Statistical analyses found were similar in the two weight ranges as follows: erythrocyte count of 1.95±0.58 and 2.19±0.64 x106/mm3, hemoglobin 9.32±2.63 and 9.94±2.84 g/dL, hematocrit of 32.44±5.78 and 33.56 ± 6.86%, plasma protein 31.86±6.14 and 32.20±6.50 g/dL and a speed erythrocyte segmentation of 7.56±5.52 and 7.94±5.17 mm/h, a mean corpuscular volume of 183.43±75.44 and 165.76±60.80 u3, mean corpuscular hemoglobin of 50.56±22.39 and 49.19±21.27 uug and concentration mean corpuscular hemoglobin of 28.68±5.39 and 29.61±5.08%, respectively. The leukocytes referent for the first range is 2.11±1.03 and for the second range it is 2.18±0.97 cells x105mm3, used to evaluate the white line as: thrombocytes, lymphocytes, neutrophils, monocytes, eosinophil's and basophils. For the degree of association between each of the variables, there is a positive correlation between weight, length and height, as well as between length and height. For the hemoglobin level a positive correlation was obtained with neutrophils and CHCM, VCM erythrocyte index also related with HCM positively and negatively to the total number of erythrocytes, CHCM index has positive correlation with neutrophils, total leukocytes were correlated with differential count of thrombocytes, lymphocytes and neutrophils. Finally there was a positive correlation between thrombocytes and neutrophils, being similar to studies reported in other environmental conditions.