The fitness of a female mosquito is determined by the success of sugar- and blood-feeding and reproduction. The behaviours associated with feeding and reproduction are mediated by chemical cues of different origin, and therefore the producers of these cues can determine the ultimate survival of a mosquito population. Mosquitoes respond to the odours of a wide range of flowers and extra-floral nectar sources for sugar-feeding. For blood-feeding, the mosquito responds to a range of emanations produced by the vertebrate host. Carbon dioxide is the compound used by most mosquitoes, but specialized species respond better to host-specific cues. Fatty acids and lactic acid are present in large quantities on the skin of humans, unlike in other vertebrates, and these compounds have been implicated in host-seeking behaviour of anthropophilic mosquitoes. One-octen-3-ol, a compound present in bovine emanations, is attractive for host-seeking zoophilic mosquitoes, especially if combined with carbon dioxide. Oviposition stimuli are present as pheromones in the apical droplets of freshly deposited eggs in several culicine mosquito species, and as attractants in emanations of organically enriched water (grass and hay infusions, water containing animal and human faeces). It is concluded that many behaviours of mosquitoes are mediated by a wide range of semiochemicals, and that the degree of sensitivity to these stimuli will therefore affect the reproductive success, and thus fitness, of these important insects.