The objectives of this study were to verify the presence of membranous vesicles (MV) in canine seminal plasma by mean of transmission electron microscopy (TEM), to describe the ultrastructural characteristics and to identify some enzymatic activity associated with them. Semen samples, collected by digital manipulation from dogs with proven fertility, were pooled and used for membrane vesicles preparation according to conventional procedures. TEM observations showed the existence of vesicular membranous structures of more or less spherical shape with different sizes. These vesicles were surrounded by a single-, double- or multiple-layered laminar membranes. The mean vesicle diameter was 117.6 ± 86.9 nm ranging from 24.4 to 716.6 nm. Enzyme activity determinations showed the presence of adenosine deaminase, 5'-nucleotidase, ADPase, ATPase, dipeptilpeptidase IV, alkaline phosphatase, total acid phosphatase and prostatic acid phosphatase, while the aminopeptidase activity was absent. In conclusion, results of this study, compatible with results from other mammals, showed for the first time the presence of MV, their ultrastructural and enzymatic characteristics in dog seminal plasma.