Colonies of Ureaplasma urealyticum serotype VI grown on agar were examined by thin sectioning technique. The arrangement of cells within the colonies was less dense and less stratified than that observed in most other mycoplasma colonies. Small, dense and pleomorphic cells were located mainly along the colony borderline towards the agar. Studies of cell morphology after 3, 10 and 15 days of growth revealed that such cells were younger than the less dense, mostly ovoid cells seen in the central area of colonies. The ultrastructure of individual cells was similar to that of Ureaplasma grown in liquid medium, as described earlier, featuring geometrical ribosome arrangements and extramembranous coat. Serial sections demonstrated the difficulties involved in discussing cell morphology on the basis of single thin sections. The advantages of the preparation method used for electron microscopy are discussed.