Ultraviolet (UV)-microbeam irradiation of a single kinetochore during anaphase generally causes all 6 of the half-bivalents in the cell to stop poleward motion within 1 min after the irradiation. The half-bivalents regain movement after remaining stopped for an average of 8.7 min, through different pairs in the same cell can resume at different times. Once movement resumes they usually continue movement until they reach the poles. As controls, to see if the effect is due to alteration of the kinetochore, we irradiated spindle fibers and chromosome arms using the same doses and wavelengths as for kinetochore irradiation. After spindle fiber irradiation, only the half-bivalent associated with the irradiated spindle fiber and its partner stop moving poleward while the other half-bivalents in the same cell are not affected. After irradiation of a chromosome arm, the movement of the two partner half-bivalents associated with the irradiated arm either slowed or moved with unchanged velocity; no other half-bivalents in the cell were affected. Therefore, only irradiation of a kinetochore stops the movement of all the half-bivalents in the same cell. We suggest that the irradiated kinetochore sends a ''stop'' signal to the other kinetochores in the same cell.