The Netherlands Antilles may be divided into: (1) The Curaçao Group (or Netherlands Leeward Islands): Curaçao, Aruba and Bonaire. (2) The St. Martin Group (or Netherlands Windward Islands): (Netherlands) St. Maarten, Saba and St. Eustatius. The latter islands are very small, forming together only 8.1 per cent of the total area of the Netherlands Antilles, and 2.2 per cent of its population. The Curaçao Group often has a desert-like aspect with a “tropical dry-forest” vegetation. Therefore on these islands the mosquito pest is nothing like so bad as it usually is in the tropics. There are few permanent breeding places, except man-made receptacles in and around the houses to store rainwater or well-water in as the Government waterworks do not always produce sufficient and adequate water. The St. Martin Group has a higher rainfall and a more abundant vegetation. In the preceding pages the morphological characteristics which are of taxonomic value have been described. Keys to the mosquitoes, their classification, their geographical distribution and their biology observed in the Netherlands Antilles have been given. Mosquitoes may be spread by automobiles, ships and airplanes on the islands. Fortunately, all airplanes from foreign airports and St. Maarten are sprayed on Curaçao and Aruba. Except this measure little was done before 1951 to control mosquitoes, except in the areas occupied by the oil companies. An anti-Aëdes aegypti campaign was initiated on Curaçao in October 1951 and on Aruba in March 1952 (residual DDT house spraying and larviciding). Because of the paucity of mosquito records of the Netherlands Antilles a rather thorough survey was made on Curaçao from 1941- 1947, while the other islands were visited only for a short time. At the moment 20 species are known from the Netherlands Antilles. Anopheles pseudopunctipennis pseudopunctipennis was found on Curaçao and rarely on Aruba, and An.albimanus once on St. Maarten, but never an indigenous case of malaria has been reported from the Netherlands Antilles. The larvae of An. pseudopunctipennis were found in earth-lined breeding places, but also frequently in manmade receptacles. Nearly all these breeding places contained clear, fresh or slightly brackish water with green algae; the majority were sunlit. Though the females of An.pseudopunctipennis attacked man, they were more attracted to animals. Culex quinquefasciatus was a common domestic pest mosquito on all of the islands. Though it often bred in earth-lined breeding places, it was found more frequently in man-made receptacles. The water was fresh or slightly brackish and usually polluted. Wuchereriasis bancrofti prevailed at a low rate on the Curaçao Group (4.2%, of which at least 2.7% was indigenous) and at a higher rate on the St. Martin Group (10.3% of which at least 5.1% was autochthonous). Elephantiasis was very rare. Aëdes aegypti was the most common domestic pest mosquito on both groups of islands. It was usually caught in clear, fresh water in man-made receptacles in or around human dwellings. The females bit in the daytime and at night. Several epidemics of yellow fever occurred in the previous century; the last one was on Curaçao in 1901. The last sporadic case occurred on Curaçao in 1914. Dengue was very common in newcomers from non-endemic areas. Haemagogus anastasionis was collected on Curaçao and rarely on Aruba. The larvae were mainly found in tree holes after occasional rains. All the breeding places contained dark brown rainwater with a layer of humus. The bite of the female is painful. Fortunately it has not been incriminated as a vector of jungle yellow fever. Besides, there are no wild monkeys on the Netherlands Antilles. Wyeomyia celaenocephala was found in various species of bromeliads on the Christoffelberg on Curaçao. The females will bite fiercely in the jungle. Uranotaenia lowii was collected from a pond on Bonaire. Aëdes taeniorhynchus was mainly caught in stagnant, sunlit beach pools with clear, dark brown, brackish water on Curaçao, and once in a well on Saba. The females are severe biters. Aëdes busckii was found in a tree hole on St. Eustatius. Psorophora cyanescens was reported from Aruba only once. Psorophora confinnis bred in rock holes and other earth-lined breeding places, and rarely in man-made receptacles on the Curaçao Group. The majority of the breeding places were temporary and sunlit, and contained clear or turbid rainwater. The females are fierce biters. They entered houses. Psorophora pygmaea was collected from a ditch on St. Maarten. Deinocerites cancer was mainly found in crab holes on both groups of islands. The water of the breeding places was turbid and brackish. Adults lived in the crab holes. Females did not bite the author. Culex erraticus was caught in clear fresh water near the airport on Curaçao. Culex americanus was found in various bromeliads on the St. Martin Group. Culex bahamensis was collected from fresh or brackish water on the St. Martin Group. Culex habilitator adults and larvae were found in crab holes on St. Maarten. Culex maracayensis was caught in earth-lined breeding places and sometimes in concrete tanks and troughs on Curaçao. The water was usually clear, shaded and fresh or slightly brackish. Culex nigripalpus was collected near the airport on Curaçao from a temporary ground pool with rainwater. Megarhinus guadeloupensis was found once in a bromeliad on Saba.