Campylobacter fetus is a gram-negative, motile, spiral or S-shaped bacterium, which induces campylobacteriosis . This disease causes decrease productivity of cattle . Although considerable research has been done on the role of C. fetus on female fertility, little is known about the effect on bulls . The aim of this work was to evaluate the effect of C. fetus subsp . fetus (Cff) and C. fetus subsp . venerealis (Cfv) on bull sperm quality . Samples of frozen semen (n = 29 straws) were each distributed into three groups: two of them incubated with the microorganism (Cff, Cfv) and a control group . The proportions of live spermatozoa, with functional membrane and true acrosomal reaction in control group were significantly (P <0.01) greater than those observed in Cff and Cfv groups . However, no significant differences (P> 0.05) were found in sperm chromatin structure among treatments . In adhesion assay, proportions of spermatozoa with adhered Campylobacter were similar for both subspecies . Results confirm that Cff and Cfv have the same ability to bind in an irreversible way to bull spermatozoa and to affect sperm quality . It is proposed that adherence could be considered as the main cause of sperm alterations, and also an important step of pathogenesis and venereal transmission.