In Azerbaijan, previously reported species Trichocephalus globulosa, T. infundibulus, T. baskakowi, T. indicus were not confirmed. Upon comprehensive research, frequent variability of the membrane of spicular sheath was detected and it was assumed that these indicated species are just different forms of T. ovis. It was also established that distal end of the membrane of specular vagina of T. ovis T. skrjabini change its forms depending on the degree of protrusion and extrusion. Due to reduction and expansion, transformation of various forms (globe-shaped, funnel-shaped, glass-shaped, etc.) in membrane of distal end of the specular vagina T. ovis takes place. Form of end of spicule changes (ends sharply, bluntly or with rounded ends). It again serves as a proof that membrane of distal end of specular vagina and form of end of spicules are volatile. In the process of microscopy study of separate body parts of the T. skrjabini, the membrane of spicule vagina in the form of light bulbs, vase and pomegranate seeds were detected. Based on numerous studies of Trichocephalus, it was concluded that in Azerbaijan two species of Trichocephalus are existed: T. ovis and T. skrjabini. Accordingly, on the basis of aforementioned, it is considered important to provide a more comprehensive image and description of morphological structure of helminths studied. The presented images allow you to easily determine the species and achieve an accurate diagnosis of the highlighted helminths.