The motion of atomic steps on Si(1 1 1) "1 x 1" is studied using in situ scanning electron microscopy to observe ultraflat Si(1 1 1) substrates that have flat terraces of 100 mum width. Atomic steps are revealed by (7 x 7)-domain decoration. Step-flow growth and sublimation processes are compared for single-step-high islands and holes. The step motion is highly symmetric for growth in comparison with Sublimation. and for islands in comparison with holes. The step motion fits well with the simple model by Burton, Cabrera and Frank. No clear asymmetry in step kinetics is observed for upper terraces in comparison with lower terraces. However, by introducing an asymmetry in the relative terrace sizes on either side of a step, the adatom flux balance is tilted, and the result is a morphological instability in step flow. (C) 2002 Elsevier Science B.V. All rights reserved.