The authors pay attention to an occurrence of the taxon Ceratophyllus sciurorum sciurorum (Schrank, 1803) (Siphonaptera) in Slovakia during the whole season. The curve of occurrence this taxon of fleas has showed two high points. The first top has been achieved in April and second one followed in October. In December the authors have noticed a period and named it "cocoon block". This course of the curve has been found at the species from family Myoxidae, mostly. On the other hand, the course of curve of the birdboxes nesters has the opossite position, the high points have been recorded in autumn. A conspicuous differences has been probably caused by different biology of nesters of Myoxidae and birds in boxes. The ratio of males to females these ecological groups of hosts was different, too.