Some problems in a correct using of the term "host specificity" for parasitic protozoans and specifically for the trypanosomatids are discussed. Results of investigation the host specificity of the trypanosomatids obtained by traditional methods are summarized. Host specificity data of some insect-associated trypanosomatids based on the identification of parasites by means of molecular methods is discussed. The subjectivity is an imminent distinctive feature of host specificity investigation in parasitic protozoans--trypanosomatids, especially in parasites of insects and plants. There is a vicious circle, when the conclusions about specificity are related with the necessity of taxonomic identification of the parasites during the process of biodiversity and ecological studies. The taxonomic position of parasite is often determined based on a data of observed hosts specificity. This is quite common in cases, when reliable morphological characters are absent, and it indeed takes place in the homoxenous trypanosomatids. The using of molecular markers only allows to reliably identify and compare the parasites, without involving the properly taxonomic data, and finally to make more objective conclusions about their host specificity. A crucial question in this kind of investigation is an obtaining of adequate and wide set of laboratory cultures (isolates) correctly reflecting the diversity of the hosts. It is always necessary to take in attention a possibility of occasional infections, which could misrepresent obtained results. About 50 cultures isolated from different hosts (mostly Hemiptera: Heteroptera) and places (mostly North Russia) have been examined by means of RAPD, UP-PCR, MLEE and cross DNA hybridization. Some of them were placed in rRNA-based molecular phylogeny. As it was found out, none natural groups of homoxenous trypanosomatids (groups of similar genomes) demonstrated a clear preferential distribution on certain insect taxon of any taxonomic rank,--species, genera, family and even order. It is postulated that the host specificity of insect-associated trypanosomatids is extremely wide.