Fruit flies (Diptera: Tephritidae) are among the most important pests damaging fruit crops growing in Tanzania. Demographic parameters for Ceratitis cosyra (Walker) and Bactrocera invadens (Drew, Tsuruta & White) are reported and compared. These include pre-adult survival and development rates, adult survival and fecundity and population parameters such as the intrinsic rate of increase, and mean generation time. Population and biological parameters of the C. cosyra and B. invadens were measured in an environmental chamber at 25°C and 30°C with 75% RH and at room temperature ranging 23–28°C. The results showed that B. invadens had shorter embryonic, larval and pupal development times than C. cosyra independent of temperature. Life expectancy of male B. invadens was significantly greater than that of C. cosyra, while life expectancy of female B. invadens was greater than that of C. cosyra but not significantly. Average net fecundity was higher for B. invadens than for C. cosyra. Both species attained their highest intrinsic rate of increase and net reproductive rate at 30°C. B. invadens exhibited higher intrinsic rate of increase and net reproductive rate than C. cosyra at all temperatures tested.