During control measures against cattle hypodermatosis low populations of warble flies with incidence of infestation (extensity, percentage) up to 2-5% persist after a rapid decrease of the degree of infestation. In order to study these low populations isolated warble fly populations were experimentally created under conditions of Central Europe. Fundamental populations observed originated from one ovipositing female in a herd of heifers and included 17, 25, 42 and 83 larvae with 11-52% of infestation and mean infestation of 1.1-1.6 larvae per infested host (intensity of infestation). With the increasing age of heifers from one to 2.5 years the numbers of fundamental populations decreased to one quarter, the extensity of infestation to one fifth, while the mean infestation per infested host remained nearly at the same level. The persistence of low warble fly populations was caused by an internal system regulating the numbers of parasites in the host populations. The distribution of larvae of fundamental warble fly population in a cattle herd agrees with the Poisson distribution as well as the negative binomial distribution. The direct significance of the results obtained for the control of hypodermatosis is pointed out.