This paper describes various aspects of oogenesis in Piaractus mesopotamicus (pacu), particularly the changes in nuclear morphology and the nuclear-cytoplasmic interactions during the early developmental stages of oocytes. Material produced in the nucleus and passed to the cytoplasm, known as “nuage”, was observed in oogonia and in oocytes. In the oocyte cytoplasm, this material gives rise to germ plasm. Vitellogenesis, chorion formation, and atresia of some follicles were also observed and were similar to those described in other teleost fish.