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Metabolizable energy intake and changes in body mass was measured in the desert gerbil Gerbillus dasyurus when parasitized by the flea Xenopsylla ramesis. We hypothesized that energy requirements for maintenance, or average. daily metabolic rate, of parasitized gerbils would be higher than those of non-parasitized gerbils. We also hypothesized that the efficiency of utilization of energy for maintenance would be the same for parasitized and non-parasitized gerbils, as this measurement is dependent mainly on dietary quality. Fifty fleas feeding on a gerbil consumed 3.68+/-1.19 mg blood, in total. This amounted to 34.3+/-1.8% of body mass of a starving flea and only about 0.17% of the blood volume of the host. The absolute amount of blood consumed by X. ramesis is the lowest reported for fleas in other studies. This suggested that the pressure of parasitism of X. ramesis on G. dasyurus in terms of blood consumed was relatively low. Both our hypotheses were confirmed. Average daily metabolic rate of the parasitized gerbils (7.75 kJ g(-0.54)d(-1)) was 16% higher than that of non-parasitized gerbils (6.69 kJ g (-0.54)d(-1)). In addition, at zero metabolizable energy intake, the parasitized gerbils lost body mass at a faster rate than the non-parasitized gerbils (4.34 vs 3.95% body mass d(-1)). The efficiency of utilization of energy in the parasitized and non-parasitized gerbils was similar.