It is quite feasible everywhere, by proper drainage, to eliminate entirely the anopheles mosquito, and in several of our towns and villages we succeeded in doing this, and could have done it everywhere if it had been thought desirable by the authorities to apply the same methods, which had been successful at these places, and previously at Havana. At stations where mosquitoes were more or less troublesome, the mosquito-catching method above described was very useful in the barracks of the laborers. In a barrack building which quarters forty or fifty men the best screening will not altogether keep out mosquitoes, if there are many mosquitoes around. Holes will continually be punched in the wire netting, but mosquitoes enter principally through the constant opening of the door, and through the door being carelessly left open. We found that at such stations we could keep malaria down by catching malarial mosquitoes. Occasionally, at a station where we had controlled mosquitoes for several years, a great swarm would appear for reasons which we could not explain. These swarms would not remain for a long time, and usually they were not made up of anopheles. While they lasted, we used the method of catching infected mosquitoes for the protection of our force. On one occasion, however, we had a large flight of anopheles. They swarmed everywhere about the station, and we could not account for them, or discover where they came from. The sanitary inspectors’ department devoted all its spare force to investigating this point, and for a considerable period Mr. Le Prince devoted all his time to the subject. He finally located the breeding area in a small swamp about a mile from the town. This swamp had existed there during the preceding years when Gatun had been comparatively free from mosquitoes. At this particular time when the anopheles were so troublesome, the engineers had begun to pump silt from the channel of the Canal into this swamp. This silt was carried by salt water, which made the water of the swamp brackish. This brackish water apparently favored the development of the anopheles, and they were produced in enormous numbers. The engineers were requested to pump sea water into the swamp area for a few days. This soon made the water of the swamp too salty for the breeding of the anopheles, and in a few days the mosquitoes disappeared from the town. It is remarkable that anopheles should have bred in this brackish water, as it is a mosquito which generally seeks fresh water. It is probable that the brackish water made some advantageous change in the food supply of the larvæ, and that when the swamp was filled with pure sea water, this food supply was stopped. This is the longest flight of anopheles which we discovered on the Isthmus.