Tiamulin, a semisynthetic antibiotic, was fed to growing pigs and its value in maintaining weight gain and feed efficiency in the face of mycoplasmal pneumonia was determined. Four treatments divided among 64 growing pigs from a conventional pig farm were used to test the effects of tiamulin administered in feed at levels of 10, 20 and 30 ppm. All pigs were artificially infected with Mycoplasma hyopneumoniae. Medication was started on the 14th day after infection (d 0) and continued for 28 d (d 0 to 28). Pigs were continued on test for an additional 14-d period for further observation (d 28 to 42) after the medicated feed was withdrawn. Tiamulin fed at 10, 20 and 30 ppm for 28 d did not cure the mycoplasmal pneumonia. The effect of tiamulin on growth performance was not the same for all periods. During the medication period (d 0 to 28), both daily gain and feed conversion were not significantly affected by dietary tiamulin concentrations. For the post-treatment period (d 28 to 42), tiamulin improved daily gain (P less than .05) and feed conversion (P less than .05) over those of nonmedicated controls, and the responses of both daily gain and feed efficiency increased linearly (P less than .05) as dietary tiamulin levels increased.