Quantitative measurements of polysomes and ribosomes of Drosophila melanogaster egg chambers, mature oocytes, and embryos were done using sucrose gradient analysis. The amount of polysomes per egg chamber increases about 20 times from stage 5 to 13, and then remains constant up to the end of embryogenesis. The percentage of ribosomes in polysomes is fairly constant during oogenesis and embryogenesis (56 ± 7%). Depending on the fly population, the percentage of ribosomes in polysomes of mature oocytes varies from 10 to 70%. It is shown that the percentage of polysomes in mature oocytes decreases with the time of retention of the mature oocytes in the ovary. Twenty-four- to thirty-six-hour-old flies kept in optimal conditions retain their mature oocytes for 2–3 hr. These mature oocytes still contain 40–60% ribosomes in polysomes. Conditions are given which allow the obtainment of reproducibly high amounts of polysomes from mature oocytes of Drosophila.