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Previous genetic and biochemical studies have led to the hypothesis that the essential mitotic bi-polar kinesin, KLP61F, cross-links and slides microtu-bules (MTs) during spindle assembly and function. Here, we have tested this hypothesis by immunofluo-rescence and immunoelectron microscopy (immu-noEM). We show that Drosophila embryonic spindles at metaphase and anaphase contain abundant bundles of MTs running between the spindle poles. These inter-polar MT bundles are parallel near the poles and anti-parallel in the midzone. We have observed that KLP61F motors, phosphorylated at a cdk1/cyclin B consensus domain within the BimC box (BCB), localiz