Microfilament integrity is needed to maintain the regular arrangement of the spindle microtubules and to guarantee the normal progression of the last syncytial mitoses in Drosophila embryo. To investigate when and how microfilaments participate in this process, we incubated permeabilized embryos with the inhibitor of actin polymerization, cytochalasin B, at different times during the nuclear cycle. Our results suggest that the correct microfilament distribution is only required for the appropriate segregation of nuclei during the 11th, 12th and 13th syncytial mitoses rather than during the 10th mitosis when the spindles are too far apart to interact. When cytochalasin B treatment was performed during the last syncytial mitoses many spindles fuse among them and the regular mitotic progression is perturbed.