Abstract: Estrone, a steroid hormone, is identified in the monoecious Cucurbitaeeous plant, Lagenaria leucantha Rusby. The content of estrone changes with developmental stages. In young seedlings with only 2–3 leaves, there was no trace of estrone. Estrone was barely detectable in seedlings with 4–6 leaves. At that time no flowers were visible. In plants with 11–13 leaves, when flowers bad appeared, the content of estrone increased greatly. It is therefore postulated that estrone may be one of the hormones controlling flowering. GA3 and CEPA were used to shift the sex expression of the plant toward the male and female directions respectively. Estrone was present in both chemically induced male and female plants, indicating estrone alone does not play any special role in sex expression. It might be the balance of the male and female hormones that controls the sex expression of the plants. It is believed that flowering and sex expression are controlled by the same set of hormones, as early postulated by Hesiop-Harrison and other workers.