Selection of an appropriate habitat with necessary and exploitable resources is one of the main tasks for any living organism.If an organism succeeds in fulfilling this task,its reward is translated into an increase or,at least,no decrease of its fitness.The evolutionary motivation of parasites does not differ from that of free-living organisms.Parasites have to carefully select their host organisms at both evolutionary and ecological scales.An individual parasite has to be able to locate and identify an individual of an appropriate host species,sex and age and to distinguish it from individuals of often similar but less appropriate or even inappropriate cohorts.Here,we report the results of investigation of the ability of fleas (Siphonaptera) to distinguish between hosts of different rodent species,genders or age categories presumably using odour cues.We will demonstrate that the ability to distinguish and to select an appropriate host individual results in fitness rewards.