Fruit flies (Diptera: Tephritidae) are among the most notorious pests damaging fruit crops grown in Tanzania. Demographic and life history parameters of Ceratitis cosyra (Walker) and Bactrocera invadens (Drew, Tsuruta and White) reared on artificial diet were investigated at Sokoine University of Agriculture laboratory in 2010. Demographic and life history parameters of C. cosyra and B. invadens were measured in an environmental chamber at 25 0 C and 30 0 C with 75% RH to determine pre-adult survival and development rates, adult survival and fecundity and life history parameters such as the intrinsic rate of increase, the mean generation time and stable age distribution. The design of the experiment was split plot arranged in a Complete Randomized Design (CRD) with three replications. There were two sources of variations: Fruit fly species (being the main- factor) and temperatures (being the sub-factor). The effect of temperature and species on eggs laying and survival were tested by two-way analysis of variance (ANOVA). The Least significance difference (LSD) test was used to identify significant main effects. The analyses were performed using GenStat package. The results show that B. invadens had shorter egg incubation time than C. cosyra in all the temperatures tested. Larval and pupal development rates of B. invadens were significantly faster than those of C. cosyra independent of temperatures. Life expectancy of male B. invadens was significantly longer than that of C. cosyra while life expectancy of female B. invadens was higher than that of C. cosyra, however, the difference was not significant. The average net fecundity was higher for B. invadens than was for C. cosyra. Both species attained their highest intrinsic rate of increase and net reproductive rate at 30°C. Furthermore, B. invadens exhibited higher intrinsic rate of increase and net reproductive rate than C. cosyra at all the temperatures tested. These findings are useful for improving laboratory-rearing methods, predicting the fly’s population dynamics, with a view to developing appropriate fly control and management strategies.