The turbidity formed as ram spermatozoa swam up into a clear medium was measured using a colorimeter and this was used as an objective index of sperm motility. Wavelengths of 470–475 nm yielded higher optical densities (OD) than did 520–640 nm. The recommended distance between the light beam and semen–medium interface, and operation temperature were 5 mm and 30–37°C, respectively. Of seven media tested, Tris–glucose was preferred on the basis of results, availability and cost. Diluting semen before testing reduced OD, whereas increasing semen concentration (0.6 to 2.4 × 109 spermatozoa ml−1) and the total number of spermatozoa used increased OD. OD gradually increased with time in all experiments (P < 0.001). High positive correlations between OD and known percentages of motile spermatozoa (0.82–0.91 at from 5 to 20 min, all P < 0.001) suggest that this swim-up technique, which is objective and inexpensive, is valid for measurement of sperm motility in rams.