Bipolar kinesin-5 motors, essential in diverse organisms, can generate positive sliding forces between overlapped interpolar microtubules to push mitotic spindle poles apart. BMK-1, the sole Caenorhabditis elegans kinesin-5, is not essential. We have determined, by tracking pole movements in bmk-1
mutant C. elegans embryos, that BMK-1 actually resists pole separation during anaphase. This provides in vivo evidence that kinesin-5, when challenged by fast pole separation forces, can serve as a rate-limiting brake for interpolar microtubule sliding.