Until recently, there has been no consensus of opinion as to that D.repens females can develop in the accidental host (man). The role of man is regarded as a biological dead end for Dirofilaria that are unable to develop the adult stage; human Dirofilaria migration is considered to be larval by the type of larva migrans. Studying these matters on a specific material of 140 cases of invasion by Dirofilaria that had been eliminated in patients in different seasons and the length of their bodies measured showed that the maximum number (51.4%) was mature females, 120 to 190 mm in size; 35.7 and 12.85% had a length of 70-115 and 40-65 mm, respectively, which corresponds to immature young and juvenile specimens. This suggests that Dirofilaria normally develop and achieve the adult stage in the accidental host. A correlation was found in the sizes of the body and the time of D. repens development. Thus, the human organism is not a biological dead end for Dirofllaria. The migration of adult Dirofilaria should be appraised as the imago-migrans phenomenon: As the particular diagnostic sign, it will aid a physician to establish a correct parasitiological diagnosis at the early stages of the disease. In recent years, there has been new evidence in the diagnosis of dirofilariasis that microfilaria have been detected in the skin tumor puncture specimen and blood of the patients. In this connection, it is conceivable that man may be an actual source of mosquito infection even in the presence and paucity of mature males and females.