Ciliary growth relies on the delivery of tubulin dimers to assemble the axoneme. It has been shown that this tubulin movement is achieved through both motor-based intraflagellar transport (IFT) and diffusion, although the relative contributions of each method are still unknown. In this study, Karl Lechtreck and colleagues (Craft Van de Weghe et al., 2020) propose that diffusion might be the main supplier of tubulin during axonemal assembly and growth. Making use of regenerating and full-length cilia in Chlamydomonas, the authors confirm in vivo that the IFT–tubulin interaction occurs through the acidic E-hook of β-tubulin (βE-hook) and the IFT74–IFT81 tubulin-binding module in the IFT machinery. The βE-hook is important for both processivity and frequency of tubulin transport by IFT. IFT of an E-hook-deficient β-tubulin tagged with GFP is greatly reduced (by ∼90%) and almost non-existent when this variant is expressed in a mutant strain lacking the IFT81 tubulin-binding domain. However, in both cases, a large pool of E-hook-deficient GFP–β-tubulin is still detected in the axoneme, which does not correlate with the observed decrease of GFP–tubulin IFT. Live imaging analysis also revealed that the rate of GFP–β-tubulin entering cilia is higher through diffusion than IFT. Altogether, these data point to a more prominent role for diffusion in delivering tubulin for axoneme formation than previously expected, although ciliary assembly still enforces the need for active transport.