ABSTRACT. Barros R.C. do R., Almosny N.R.P., de Mesquita E. de F.M. & Torres Filho R. de A. [Hematological aspects of tilapia, Oreochromis niloticus (Osteichthyes: Ciclidae), captured from cultivated areas of Piraí in the State of Rio de Janeiro, Brazil]. Observação sobre os aspectos hematológicos de tilápia, Oreochromis niloticus (Osteichthyes: Ciclidae), capturada em cultivos na região de Piraí, Rio de Janeiro, Brasil. Revista Brasileira de Medicina Veterinária, 34(3):183-187, 2012. Departamento de Zootecnia, Laboratório de Aqüicultura, Faculdade de Veterinária, Universidade Federal Fluminense, Rua Dr. Vital Brazil Filho, 64, Niterói, RJ 24230-340, Brasil. E-mail: email@example.com The hematological values give a diagnostic support to find out the type of disease that is spreading into the population or individual, and also the evolution of it. Studies in blood cells of several species are rare. This research has the objective of a comparative study on fishes’ hematology, in order to obtain patterns to be used in tilapias, Oreochromis niloticus, examinations commercialized and cultivated in Piraí region, Rio de Janeiro state. Fifty blood samples were analyzed and hematological determination was done. The hematocrit (Ht or HCT), hemoglobin, hematimetric reference values, VHCM and CHCM show variation among the samples. It was observed that thrombocytes, monocytes and lymphocytes were high. The hematocrit was the minor average variation index.