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Summary The present paper reports a series of observations on the habits of the adult mosquitoes dwelling in the llanos, the grass land of Eastern Colombia. The observations summarized here were made during the rainy season, in the months of June, July and November, 1948 and June, 1949. Captures of adult mosquitoes were made by means of stable traps and with muslin tents covering an area of 4 sq. m. of ground. Catches of flying mosquitoes were also made using nets of strong muslin, fixed to a moving vehicle. The tent captures were made in series along a straight line stretching from a breeding place to the middle of an expanse of savanna. They were all made during daytime, between 7 A.M. and 5 P.M. The grass under the tent was sprayed with citronella in order to disturb the resting mosquitoes, which were then easily caught on the sides and cover of the tent. Each trial took between 15 and 20 minutes time. In a total of 104 captures, an average mosquito density of 11.9 mosquitoes per trial was obtained. Culicines formed the bulk of these captures. The average anopheline density was 1.2 mosquitoes per trial. As the type of tent used covers 4 sq. m. of ground, this means a density of roughly 3 million mosquitoes per sq. km. and 300,000 anophelines per sq. km. Males and females of A. peryassui, A. pessoai and A. parvus were found resting in the grass. A male of A. punctimacula was also found in these captures. The commonest culicine species were Culex chrysonotum, Culex (Melanoconion) sp. and Psorophora confinnis. Direct observation showed that usually the mosquitoes were resting in the grass at no more than 10 cm. above the ground. A total of 570 male culicines and 536 females were caught in the 104 trials, which may be taken as an indication of a similar span of life among both sexes. Total number of anophelines was 30 males and 98 females, perhaps too small a sample to draw any definite conclusions as regards survival. The average fertilization rate among both groups was 81 per cent, which indicates that mating occurs in all probability shortly after the emergence of the adults. In the course of these studies it was noticed that usually a mosquito breeding place can be found anywhere in the llanos at no more that 1,000 to 1,500 m. distance. The maximum distance recorded from a breeding place was 1,800 m. Female mosquito density proved to be nearly uniform across the savannas, but male mosquito density was found to be higher near the breeding places than far away from them. Such findings demonstrate that, at least during the rainy season, any part of the savannas of the llanos is within the effective flight range of the species living there. A series of parallel stable trap captures made in the vicinity of a breeding area and at a distance of 1 km. gave a mosquito density approximately the same for both locations. Stable trap captures yielded a good number of anophelines but they gave inadequate information as regards the culicines. Tent captures and captures from a moving vehicle, are considered superior to the ordinary way of trapping to obtain direct information on the real mosquito density and on the composition of the mosquito fauna in a savanna country. Biting activity of mosquitoes was measured over a 24 hour cycle by periodical removal of the mosquitoes entering a baited stable trap. Maximum activity took place shortly after sunset, continued during the night and had a slight increase before sunrise. Mosquito activity during the day was practically nil. Direct observations were made of sexual activities. Swarms of culicines were seen over the savannas as early as 5 P.M., but mating of anophelines was only observed on two occasions at 7 P.M.; swarming males captured then proved to be A. peryassui.