The spindle pole body (SPB) is the principal microtubule organizing center of budding and fission yeast. We have examined SPBs and their associated microtubules from both organisms, using electron microscopy and three-dimensional reconstruction techniques, to identify the structural changes that accompany progression through the cell cycle. In this report, we compare these changes in the two kinds of yeasts and present a model for how microtubules get into a closed nucleus.