- Summary 1Two immunological tactics, a constitutive response that is always present, and an induced response that is only employed after an invader has been recognized, have evolved in animals as a defence against parasites. 2The energy requirements, body mass change and blood parameters of flea-parasitized and nonparasitized gerbils (Gerbillus andersoni) were measured and compared with published data for a close relative (Gerbillus dasyurus). G. andersoni possesses a constitutive immune response that could require additional maintenance costs, whereas G. dasyurus develops an immune response only after being attacked by fleas. We therefore predicted that G. andersoni has higher energy requirements than G. dasyurus when both species are parasite-free. However, we also predicted that the immunological ‘readiness’ of G. andersoni makes it less susceptible to flea infestation than G. dasyurus. 3Energy requirements for maintenance were estimated by offering different levels of metabolizable energy to parasitized and nonparasitized animals and measuring changes in their body mass. At the end of the experiment, blood samples were taken from all rodents and haematological and biochemical analyses were done. 4Adjusted energy requirements per unit body mass of nonparasitized G. andersoni were higher than those reported for G. dasyurus. Also, there was no difference between energy requirements, body mass change and blood parameters of parasitized and nonparasitized G. andersoni in contrast to the pronounced differences found for parasitized and nonparasitized G. dasyurus. However, parasitized G. andersoni did not have lower energy requirements than parasitized G. dasyurus as predicted from the second prediction. 5The results illustrate the energy costs and benefits related to each immunological strategy and suggest that parasites can cause other nonenergetic costs to their hosts. The two strategies are most likely a reflection of interspecific differences in probability of flea attacks.