Two groups of 21 mixed-breed heifers were wintered on separate permanent pastures. Each heifer from one group was administered a sustained-release morantel bolus on October 7 (day 0), and the other group remained as untreated controls. Body weights were determined and fecal samples were taken at 28-day intervals. At the onset of the trial and at every 56 days, 6 heifers were removed from each group for slaughter to determine the developmental stages and the number of gastrointestinal nematodes. In addition, 3 tracer calves that were free of gastrointestinal nematodes were released on each pasture for 28 days at the beginning of the trial and after the last experimental-group calves had been removed. The 6 calves slaughtered on day 0 of the trial had a mean of 5,544 gastrointestinal nematodes. Tracer calves acquired 31,143 and 30,530 gastrointestinal nematodes from the pastures containing the treated and control heifers, respectively. Throughout the trial, the number of nematodes in the control calves increased at each sampling date (mean, 126,168 worms), whereas the mean number of worms in the treated heifers was 45,458. Tracer calves placed in the pastures after the 168-day trial acquired significantly more worms (9,632 vs 2,899; P < 0.05) from grazing the pastures with control heifers than from grazing the pastures with treated heifers. Counts of eggs per gram of feces were significantly different (P < 0.01) between the 2 groups from day 28 through day 112. Beginning at day 28, mean weight gain in the treated calves (45.1 kg) was significantly (P < 0.01) greater during the trial than was the mean weight gain for the control calves (2.5 kg). The use of a sustained-release morantel bolus in calves on winter pasture in the southern United States proved to be of value on the basis of fewer nematodes acquired and improved weight gains.