While studying the population ecology of cattle warble flies (Diptera, Hypodermatidae) in Czechoslovakia, the Soviet Union and Mongolia a firm internal regulatory system, determining the population density of parasites and operating on the principle of negative feedback, was demonstrated. The negative binomial distribution proved to be a suitable model in other parasite groups as well. The internal regulatory systems are common consistent patterns in the parasite-host relationship with specific parasites. These consistent patterns and the methodical approach to them are of wider importance not only for parasitology in particular, but for the research of ecosystems in general. This trend of research can be therefore considered to be momentous, comparable with other concepts in parasitology.