AbstractCytoskeletal inhibitors were used as probes to test the involvement of microtubules and actin microfilaments in the development, motility, and shape maintenance of the pseudopodial networks (i e, reticulopodia) of the foraminifers Allogromia sp strain NF and Allogromia laticcllaris. Agents that disassemble cytoplasmic microtubules (cold, colchicine, and nocodazole) arrest all movement but have variable effects on reticulopodial shape. Electron microscopy reveals a granulofibrillar matrix but few, if any, microtubules in these motility‐arrested reticulopods. Allogromiids treated with cytochalasin B or D lose substrate adhesion and undergo dramatic changes in shape and motile behavior, highlighted by the coalescence of reticulopodial cytoplasm into irregularly shaped bodies with chaotic motility. Serial semithick sections of such preparations, viewed by high‐voltage electron microscopy, document a striking rearrangement of microtubules within these cytochalasin‐induced bodies. All aspects of cytochalasin‐altered motility are completely inhibited by colchicine. Actin is present in reticulopodia, as determined by staining with rhodamine‐phalloidin; this staining is not observed in cytochalasin‐treated organisms. These data provide compelling evidence that microtubules are required for reticulopodial motility. An actin‐based cytoskeleton is thought to play a role in maintaining shape, mediating pseudopod/substrate adhesion, and coordinating the various microtubule‐depe