The affect of nematode infestation on the reproductive activity of flea females of genus Coptopsylla, was studied in the Central Kyzyl Kum (43–40° N) in 1993–1997. 1783 females of C. lamellifer rostrata, C. bairamaliensis and C. olgae were collected from burrows of gerbils and animals themselves at 200 points and dissected in laboratory. Coptopsylla, infested with the nematodes, were detected at 70 collection points (35 % of total). To collect Coptopsylla material the same points were used several time throughout the study. Infested fleas occurred at the same locations during the whole period of investigations. A certain type of correlation was detected between the extensiveness of nematode infestation of C. lamellifer and the reproductive activity of the fleas. Thus, in the 1st decade of November, when the extensiveness of infestation reaches the maximum, the relative number of mature female fleas decreased 1.7 times compared to the third decade of October. From the 1st to the 2nd decade of November, when the number of infected spesimens decreased (1.3 expressed as percentage), the reproductive activity of females increased. Proportion of infested C. bairamaliensis was significantly lower than that of C. lamellifer – no more than 10 %. Dissection of C. olgae revealed single infected specimens only in certain years. The nematodes were found only in 8 specimens, representing 5.3 % of prepared. In the abdominal cavity of females of C. lamellifer, C. olgae and C.bairamaliensis, only gamogenetic female nematodes and hundreds of larvae were detected. They belonged to the genus Psyllotylenchus Poinar et Nelson, 1973 (Allantonematidae). Parthenogenetic females were not found.