Feeding rumen-protected isomers of conjugated linoleic acid (CLA) to fresh dairy cows improves fertility by reducing the postpartum interval to first ovulation and enhancing blood IGF-I levels. To our knowledge, there are no studies on the effect of CLA dietary supplementation on semen quality and few on the effect of CLA addition to semen extender during freezing. The objective of the study was to evaluate the effect of dietary CLA supplementation and of CLA addition to semen extender on semen quality and freezability in bovines. Fourteen bulls blocked by breed, age, BW and BCS were randomly assigned to 2 groups: control (CTL) and CLA (50g/day). CLA was supplemented for 10 weeks and samples of blood, seminal plasma and ejaculate were collected twice a week on −2, −1wks before supplementation, 4 and 5wks (during supplementation) and 11,12wks (after supplementation). Blood and seminal plasma were analyzed for IGF-I; ejaculate from each bull was frozen in 6 subgroups to which CLA isomers were added to the semen extender as follow: CTL (no addition), CLA9,11 50uM, CLA9,11 100uM, CLA10,12 50uM, CLA10,12 100uM, CLA mix. The sperm was analyzed for %motility, %progressive, average pathway velocity, straight-line velocity, curvilinear velocity, beat cross frequency, straightness, linearity, using the CASA system; morphology trough eosin-nigorsin staining; viability, mitochondrial activity and oxidative stress using the flow cytometer with live/dead viability kit, mitoprobe JC-1, 2′,7′-dichlorodihydrofluorescein diacetate and TO-PRO-3 dyes. Data were analyzed by ANOVA for repeated measures. Preliminary results show that dietary CLA supplementation decreased the total volume of the ejaculated (P < 0.05), increased sperm concentration (P < 0.05) and increased plasma and seminal plasma IGF-I levels (P < 0.001) compared with the CTL. Sperm from CLA bulls had increased beat cross frequency (P < 0.05) and decreased oxidative stress (P < 0.1). The results support the authors' hypothesis of dietary CLA supplementation improving semen quality and bull performance. Investigation on the effect of CLA addition into the semen extender and its interaction with CLA dietary supplementation is ongoing.