Tsetse (/ˈsiːtsi/ SEET-see, US: /ˈtsiːtsi/ TSEET-see or UK: /ˈtsɛtsi/ TSET-see), sometimes spelled tzetze and also known as tik-tik flies, are large biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus Glossina, which are placed in their own family, Glossinidae. The tsetse are obligate parasites that live by feeding on the blood of vertebrate animals. Tsetse have been extensively studied because of their role in transmitting disease. They have a prominent economic impact in sub-Saharan Africa as the biological vectors of trypanosomes, which cause human sleeping sickness and animal trypanosomiasis. Tsetse are multivoltine and long-lived, typically producing about four broods per year, and up to 31 broods over their lifespans.
Tsetse can be distinguished from other large flies by two easily observed features. Tsetse fold their wings completely when they are resting so that one wing rests directly on top of the other over their abdomens. Tsetse also have a long proboscis, which extends directly forward and is attached by a distinct bulb to the bottom of their heads.
Fossilized tsetse have been recovered from Florissant Fossil Beds in Colorado, laid down some 34 million years ago. Twenty-three extant species of tsetse flies are known from Africa.
Tsetse were absent from much of southern and eastern Africa until colonial times. The accidental introduction of rinderpest in 1887 killed most of the cattle in these parts of Africa and the resulting famine removed much of the human population. Thorny bush ideal for tsetse quickly grew up where there had been pasture, and was repopulated by wild mammals. Tsetse and sleeping sickness soon colonised the whole region, effectively excluding the reintroduction of farming and animal husbandry. Sleeping sickness has been described by some conservationists as "the best game warden in Africa".