A 3·5 MHz linear transducer was used to scan the intercostal spaces of 30 healthy cows from dorsal to ventral on the right side, and the appearance, dorsal and ventral limits and size of the omasum were determined in each intercostal space. The size of the omasum determined ultrasonographically was compared with that determined postmortem. The distance between the omasum and the peritoneum of the lateral body wall was also determined electronically in each intercostal space by means of the two cursors. In the sixth to 11th intercostal spaces, the omasum had a crescent shape, with only the wall closest to the transducer visible as a thick echogenic line. The dorsal limit of the omasum appeared from cranial to caudal as the upper part of a circle, whereas the ventral omasal limit appeared as the lower part of a circle. The mean (sd) size of the omasum varied with the intercostal spaces from 16·3 (1·5) cm to 56·9 (10·0) cm; it was greatest in the ninth intercostal space and decreased cranially and caudally. The omasum was closest to the right abdominal wall in the eighth and ninth intercostal spaces, and was immediately adjacent to these spaces in 22 and 20 cows, respectively. There were significant correlations between the size of the omasum determined ultrasonographically in the ninth intercostal space and the weight, volume and largest and smallest diameters of the omasum determined postmortem, with correlation coefficients (r) between 0·38 and 0·55.