Parainfluenza virus-3 is the most common etiological agent in mixed respiratory diseases of calves with high concentrations of animals. The more severe course of the disease is observed with complications from bacterial or other viral infections. This article presents the results of clinical and epizootic, serological, virological and molecular genetic studies. A cytopathogenic agent was isolated from a pathological material taken from patients with respiratory diseases of calves in a BEK cell culture. Based on the results of serological and molecular biological studies, this isolate (“LD-9”) was identified as the parainfluenza-3 virus in cattle.