This subject has been discussed very fully by Dr. T. A. M. Nash and the writer in two recent papers (Bull. Ent. Res. 24, 1933, pp. 135, 475–477). There is, however, one point which it is felt should be made clear. Dr. Nash states that the writer, “working in the Western Kondoa fly-belt, finds the more normal type of obvious concentration. In the dry season tsetse become more numerous in Berlinia-Brachystegia woodland immediately surrounding ‘vleis.’ Owing to the small size of these ‘vleis,’ which are only about 75 yards across, the fly appears to an observer to be concentrated in them; in reality they are probably living in the shade of the surrounding weeds and fly out to attack the investigator. Passing through Berlinia woods in this country one finds fly more numerous on the edge of “vleis,” probably because the trees are almost non-deciduous owing to sub-soil water.”