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We studied whether (a) previous infestation of a rodent host with fleas and (b) the reproductive effort of fleas affect the rate of CO2 emission in two flea species, host-specific Parapulex chephrenis and host-opportunistic Xenopsylla ramesis when feeding on their typical and atypical rodent hosts. We measured the rate of CO2 emission in preovipositing and ovipositing female fleas fed on either pristine or previously infested Acomys cahirinus (typical host of P. chephrenis) and Dipodillus dasyurus (typical host of X ramesis). When P. chephrenis fed on a typical host, its massspecific rate of CO2 emission was not affected by previous infestation of a host, whereas when this flea fed on the atypical host, its rate of CO2 emission was higher when a host was previously infested. This was manifested, however, mainly during the oviposition period. The rate of CO2 emission by X ramesis feeding on pristine hosts was significantly lower than in conspecifics feeding on previously infested hosts, independent of host species. Both. flea species feeding on their typical hosts emitted CO2 at similar mass-specific rates during preoviposition and oviposition, except for P. chephrenis feeding on D. dasyurus, which increased its rate during oviposition. There was no effect of the number of eggs produced per female on the rate of CO2 emission during oviposition. J. Exp. Zool. 311A:389-398, 2009. (C) 2009 Wiley-Liss, Inc.