This study compares the in vitro ultrastructure of Streptomyces somaliensis with the grain formation seen in vivo. Cultured forms were cocco-bacillary with thick cell walls and septa. Individual cells stained with PAS.TCH and Alcian blue. Grains were largely composed of an electron-dense fibrillar matrix surrounding clear areas, some of which contained organisms. The ultrastructural appearances of the latter were not significantly different from those seen in vitro although the cell wall was less well defined and did not stain with Alcian blue, and there were more intracellular lipid droplets. Host cells surrounding the mass were incorporated into the structure of the grain, a process associated with cell death. The grains of S. somaliensis appear to be derived from both host and actinomycete elements. Compared to other mycetoma agents the mechanism of grain formation by this organism is distinctive.