Polyamines are important in the growth, division and differentiation of many cell types, including trypanosomes. The ultrastructure of culture forms of Trypanosoma granulosum was examined following growth in a modified semidefined medium (controls) and in the same medium with added polyamine biosynthesis inhibitors (DFMO, MGBG and Berenil). Untreated trypanosomes had ultrastructural features in common with other cultured flagellates. Those treated with DFMO were generally more rounded in contour. Cytoplasmic vacuoles and swollen mitochondrial membranes suggested osmotic imbalance, perhaps resulting from compromised membrane integrity. An anticlockwise arm dividing subtubule B of the peripheral doublets of the flagellum was noted in some trypanosomes treated with 20 mM DFMO. Vacuolation was also induced by MGBG and flagellar division occurred more frequently at 0.2 mM than in controls. With 1 mM MGBG, mitochondria were difficult to discern and kinetoplasts were disrupted. With 0.2 mM Berenil, vacuoles, swollen mitochondria, membranous whorls, additional microtubules underlying subpellicular tubules and disaggregated kinetoplasts were noted. With 1 mM Berenil, much of the cell structure was destroyed, although the pellicle, flagellum and paraxial rod remained intact. The present study illustrates the importance of hnctional polyamine synthetic pathways for the integrity of membranes, mitochondria, kinetoplasts and possibly microtubules in cultured T. granulosum.