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The role of water balance capabilities of fleas was examined in desert habitats. The fleas studied were Xenopsylla ramesis Rothschild and Xenopsylla conformis Wagner. Both fleas occur on Sundevall’s jird, Meriones crassus, in the Negev Highlands of Israel but in different macro- and microhabitats. Because M. crassus occurs in several habitats of the highlands, it was used as a model for investigating the effect of habitat parameters on species composition of fleas within a host species. Water balance parameters investigated were the range of humidities over which active water uptake occurs in the larvae and prepupae of X. ramesis and X. conformis. Critical equilibrium humidity estimates were close to 65% RH for larvae and prepupae of both species. Water loss rates were determined for each life stage, except eggs, and represented water loss from cuticular, respiratory, and other body openings under conditions of little or no bulk air movement. When converted to a proportional rate (1.44-2.37% mass loss h(-1)) water loss rates did not differ significantly between stages or species. Thus, geographic separation of X. ramesis and X. conformis could not be explained by any difference in water uptake capabilities or water loss rates. Other factors that may be important include interspecific competition for resource availability among larval fleas and effect of soil texture on cocoon construction.