Successful reactivation in vitro of anaphase B has recently been achieved with mitotic spindles isolated from the diatom Stephanopyxis turris. When a population of isolated spindles was studied indirectly by using immunofluorescence, nearly all of them were found to have elongated; however, when studied directly by using video microscopy, only a small proportion of spindles elongated. We report here conditions that allow nearly all of the spindles to elongate when observed directly with video microscopy. These direct observations validate previous ones made using indirect immunofluorescence. In addition, we find that the isolated spindles elongate with a linear rate, that the elongation is unchanged after the chromatin surrounding the spindles is digested with DNase I, and that during elongation a phase-dense matrix may accumulate in the spindle midzone.