Abstract: τ proteins are microtubule‐associated proteins that promote microtubule polymerization in vitro and in vivo. They are a family of neuronal proteins with apparent molecular weights in the range 50,000–68,000 determined by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. Recently, a new member of this family has been described and its cDNA has been cloned. It has an apparent molecular weight of 116,000 and has been called high‐molecular‐weight τ (HMW τ). All the τ proteins are encoded by a single gene, which undergoes complex alternative splicing. In the present study, we have cloned into the baculovirus a cDNA fully encoding HMW τ as well as a truncated cDNA encoding a protein beginning 13 amino acids in front of the τ microtubule‐binding domain. HMW τ‐recombinant‐virus‐infected Sf9 cells overexpressed HMW τ, which induced the polymerization of microtubules and the formation of long cellular processes similar to those induced by low‐molecular‐weight τ (LMW τ) overexpression. Process cross sections revealed a larger spacing (≈35 nm) between microtubules when induced by HMW τ than when induced by LMW τ (≈20 nm). The truncated construct also induces processes, where microtubules were packed far more closely together (≈10 nm). Although branching did not occur in processes induced by intact τs, 10% of the processes induced by the truncated τ protein branched.