BubR1^{A1204K.T:Avic\GFP-S65T} flies are viable and fertile. Only 1% of neuroblasts are aneuploid (as in controls). Live imaging of BubR1^{A1204K.T:Avic\GFP-S65T} neuroblasts reveals a tendency to prolong prometaphase and significant problems in spindle function. The time elapsing from nuclear envelope breakdown to anaphase is longer and more variable than in wild-type. Chromosomes are slow to congress to the metaphase pate and sometimes have difficulty remaining aligned. Spindle-K fibers are often far thinner than in wild-type and sometimes remain so right yp to anaphase onset. BubR1^{A1204K.T:Avic\GFP-S65T} spindle length is unstable during prometaphase and metaphase, often shrinking soon after nuclear envelope breakdown. Despite these defects, chromosomes segregate accurately at anaphase and cytokinesis ensues without obvious difficulty.