2000 males and females of Glossina palpalis gambiensis were released in the presence of a Zebu treated with flumethrin pour-on in a fly proof stable. From the first day after treatment until day 15, the flies were released at intervals of two days. The mortalities were highest during the first five days after treatment. With the exception of the first two releases the "knock down" effects were distinctly higher than the corresponding mortalities, ranging between 60% and 100% during the observation period.