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The acquired immunity was found to affect considerably warble flies during hypodermatosis of cattle. The intensity of infection and recurrence of the disease (the main factors affecting the intensity and duration of the acquired immunity) cause changes in the growth and development of larvae, their survival as well as in body mass and fecundity of adult warble flies (females).

During control measures taken against hypodermyiases a rapid drop of cattle infestation follows, but low warble fly populations persist, the extensity of invasion being up to 2--5%. In order to study these low populations experimentally isolated populations were established under conditions of Central Europe. Fundamental populations initiated by one gravid female H. bovis in a herd of young cattle consisted of 17, 25, 42 and 83 larvae, the extensity of infection being 11 to 52% and intensity of it 1.1 to 1.6 larvae per host. The persistence of low warble fly populations is due to an internal regulatory system limiting the population density of parasites in the host populations.

The method of statystical analysis of the distribution of the 2nd and 3d instar larvae of Hypoderma bovis in 54 herds (8120 heads) of cattle from Czechoslovakia and in 20 herds (1809 heads) from Mongolia has shown that in most cases the negative binomial distribution models the distribution of larvae with sufficient reliability and that the variability in parameters of the distribution is stable independent on the parts of the area.

The paper considers diseases of marals and axis deer caused by warble flies as one of the factors affecting the deer-breeding productivity. Data on the infection rate of marals with larvae of Hypoderma diana and Pharynomyia picta and axis deer with H. diana are given.

Undertaken investigations have shown that the antigen prepared from larvae of O. tarandi is diagnostically effective and stricktly specific only to this infection in the reaction of indirect hemagglutination (RIHA). The experiments have proved the possibility of practical use of RIHA as a principal method of early diagnosis of infection of reindeer caused by O. tarandi.

Immunity, especially aquired one, was found to play an important role in preventing the animals against falling ill with hypodermatosis. According to the results obtained, the intensity and duration of immunity, as the main factors of non-susceptibility, depend on the infection intensity, recurrence of the disease, periods of mortality or warble flies larvae and fatness of animals.