The tomato borer Tuta absoluta (Meyrick) (Lepidoptera Gelechiidae), is of growing concern worldwide as a key pest of tomato. In Brazil, the pest is controlled by frequent pesticide sprays, leading to quick development of pesticide resistance in the pest, to high pesticide residue levels on the fruit and to decimation of natural enemies of this and other tomato pests. In order to develop biological control programs, basic information on the biology of the pest T. absoluta is essential. In addition, potential occurrence of resistance to the pest in tomato lines might be an interesting option for control of the pest, either alone or in combination with biological control. Therefore, we studied development, sex ratio, weight, fecundity and population growth of T. absoluta at constant (25 °C) and alternating temperature (30/20 °C) on two tomato lines (Bravo and Tex 317), which were expected to have different levels of resistance to the tomato borer. Tomato line and temperature regime significantly influenced many life history parameters. Development time was generally shorter and emergence rates higher on Bravo than on Tex 317. However, egg development time was similar on both tomato lines and temperature regimes, though egg survival was much lower on Tex 317. Also larval survival was lower on Tex 317, and this lower egg and larval survival suggests antibiosis resistance. Interestingly, the sex ratio was more female biased at the alternating than at the constant temperature on both tomato lines. Another temperature effect was a longer egg to adult development time on both lines at the alternating temperature regime. The net reproductive rate (R0) and the intrinsic rate of population increase (rm) were higher on Bravo than on Tex 317, again indicating that Bravo is the most suitable host plant of the two. On the partially resistant tomato line Tex 317 population development is estimated to be much slower than on line Bravo. Also, T. absoluta larvae are exposed longer to natural enemies on Tex 317. Possibilities for biological control of this pest are discussed.