Ovaries of Anopheles triannulatus were dissected 24 h and 48 h after a bloodmeal in order to study its effect on oogenesis. Histological sections were analized by electron microscopy and the morphological changes ocurring at the oocyte - follicular epithelium and follicular well interface were observed. Structural aspects support the hypothesis that yolk proteins axe produced outside the ovary. Protein granules arise mainly by fusion of pit-vesicles formed at the oocyte surface.