We employed an improved fixation procedure for electron microscopy using ruthenium red, and found a bundle of contractile tubules inside the axopodia of the heliozoon Actinophrys sol. Upon food uptake, the tubules shorten and transform into a mass of small granules when rapid axopodial contraction occurs, suggesting that these structures are involved in the process of axopodial contraction. The relationship between transformation of the contractile tubules and accompanying disassembly of the axonemal microtubules was studied by examining the ultrastructure of the contractile tubules after disassembly of the microtubules was artificially induced by cold or colchicine treatment. Granulation of the contractile tubules was induced by cold but not by colchicine treatment. During recovery from cold treatment, granular forms of the contractile tubules became re-elongated and their initial tubular appearance was restored. These results suggest that the contractile tubules in heliozoon axopodia play a role in repetitive cytoplasmic contraction.