The persistence of DDT deposits on vegetation in the Northern Guinea Savannah zone of Nigeria was studied by chemical analysis of leaf samples and by bioassay tests with teneral females of Glossina palpalis (R.-D.). Two formulations of DDT were used, one (A) made by mixing a 75 per cent, wettable powder with water to give a 5 per cent, suspension, the other (B) made by adding a household detergent to A at the rate of 3·8 g. per gal. suspension. They were applied to foliage by knapsack sprayer in August 1964, about the middle of the rainy season.

It was noted that the particles deposited from B were finer and more evenly distributed over the leaf surface than were those from A. Chemical analysis of leaf samples showed that the deposit from B was initially the higher. Both deposits declined greatly during the first six weeks after application, coinciding with the period of heaviest rainfall. Those from A and B reached 49·8 and 46·2 per cent., respectively, of their initial levels at the end of the second week, and 9·2 and 5·2 per cent, at the end of the sixth. From then to the end of the sixteenth week, when the observations ended, no further decline was observed. In the tests with G. palpalis, deposits from A and B caused mortalities of 60.3 and 66.8 per cent., respectively, in the fourth week and 79·4 and 82·8 in the fifth, no mortalities below the latter being subsequently observed up to the tenth week.

It is considered that rainfall was the main factor responsible for removing the deposits from foliage. After the end of the rainy period, losses of the deposits became negligible. Deposits from B at first declined faster than those from A. It is considered that this was due to the faster removal by rain of the smaller particles.

Heavily weathered deposits were highly toxic to G. palpalis under the test conditions, and it seems likely that under natural conditions of contact they would have exerted still greater toxicity.