The effects of cyanocobalamin on the protein, RNA and DNA contents of ovary of silkworms, Bombyx mori L., were investigated. Two sets of experiments were performed, one set dealing with the changes on some selected days in protein, RNA and DNA contents after a single injection of different doses of cyanocobalamin (0.25, 0.5, 1, 2 and 5 μg/g of body weight), given on 2nd day of fifth larval instar (day 0), and the other dealing with the day-to-day variations of these cellular constituents and the effect of a particular dose (2 μg/g) of cyanocobalamin on the daily changes of the substances. In the first set of experiments, the enhanced protein content of ovary over the control value was found on day 8 (2nd day of pupa) and day 13 (adult stage) with 1, 2 and 5 μg of cyanocobalamin/g. Cyanocobalamin at the dose of 0.25 and 0.5 μg/g was ineffective in causing any change in protein content. Increased RNA content was noted in comparison to the control on days 3 (5th day of fifth instar), 8 and 13. In case of DNA content of ovary, the enhancement was observed on days 8 and 13 with 1, 2 and 5 μg/g doses. It was further evident from day-to-day changes that protein, RNA and DNA in ovary followed a specific individual pattern of changes during the developmental period under study. Cyanocobalamin at the dose of 2 μg/g increased protein content of ovary from day 5 to day 13, RNA content from day 3 to day 13 and DNA content from day 4 to day 13 without altering the specific pattern of variation of these cellular constituents. It was further observed that ovarian weight and egg number increased with different doses (0.5–5 μg/g) of the vitamin.