Morphological development of the gonads during the three larval instars of Dermatobia hominis is described. It was impossible to individualize the gonads on the first (L1) and second (L2) insars, by dissection under stereoscopic microscope. They appear in whole sections, as a small cellular cluster surrounded by a tunica, not exceeding 30 µm in diameter in L1, and 60 µm in L2. Histologically they show two types of cells, one with large round nuclei and other with small ovoid ones; these latter are located under the tunica and among the former. In the third instar the larvae grow considerably and gain weight from 50 to 900mg. In larva weighting between 100 to 200mg, under the stereoscope the gonads appear as a small dot; in larvae weighting between 250 to 350mg they appear as two transparent spheres of approximately 100 µm of diameter. The anatomical distinction between testicle and ovary ins only possible in larvae weighting 400 mg or more. This anatomical distinction is followed by rearrangements of the different types of cells.