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Fleas consume and digest blood from their hosts. We hypothesized that the energy costs of digestion of blood by fleas is dependent on the host species. To test this hypothesis, we studied CO2 emission, a measure of energy expenditure, during digestion of a blood meal taken by Parapulex chephrenis from a preferred (Acomys cahirinus) and a non-preferred (Gerbillus dasyurus) host. We predicted that the energy cost of digestion would be lower for A. cahirinus blood than that for G. dasyurus. Male and female fleas consumed similar amounts of blood per unit body mass, independent of host species. Our prediction was supported in that fleas expended significantly more energy digesting blood of G. dasyurus than blood of A. cahirinus. We also found CO2 emission rates of fed fleas were higher than those of unfed fleas and differed significantly among stages of blood digestion when a flea fed on G. dasyurus but not when it fed on A. cahirinus. When fed on G. dasyurus, fleas spent less energy during earlier than later stages of digestion.