Fluorescent brain tubulin, injected into living cells of the green alga Nitella pseudoflabellata and the higher plant Tradescantia virginiana, incorporates into the cortical microtubules, allowing these structures to be observed. With confocal laser scanning microscopy, clear images of microtubules were recorded and changes in microtubule patterns documented. After injection, fluorescent lengths of microtubules appeared within a few minutes and their number and length increased rapidly to a “steady state” over the first 15 min. In many instances, fluorescent microtubules could still be detected several hours after injection. In the cells examined, microtubules are arranged as an array of separate units only occasionally displaying close association or accurate co-alignment with neighboring microtubules. In what we perceive to be the steady state condition, some microtubules remain relatively static, while others undergo rapid changes in length or small translocations. We also document what appears to be bidirectional microtubule elongation during postdepolymerization assembly. © 1993 Wiley-Liss, Inc.