Spindle formation is examined in two cryptophycean algae,Cryptomonas sp. andChroomonas salina. During interphase a few (< 10) microtubules extend from the flagellar base pair at the anterior of the cell to the nucleus at the posterior. The beginning of prophase is signalled by the replication of the flagellar bases and the proliferation of the microtubules associated with them. The microtubules extend from the amorphous material surrounding the bases and from various flagellar roots. Microtubule numbers continue to increase as the nucleus migrates toward the cell anterior and the replicated flagellar base pairs migrate apart, eventually coming to lie just anterior to and on either side of the nucleus. At this stage more than 150 microtubules may be associated with each flagellar base pair and most of this number is associated with one of the two flagellar bases in each pair. The nuclear envelope then breaks down as the microtubules apparently disassociate from the flagellar bases and enter the nucleoplasm. Fewer than 10 microtubules are left in association with each flagellar base pair. Two spindle configurations are found between nuclear envelope breakdown and metaphase. In one microtubules extend in all directions although there is a tendency for them to be parallel in local regions particularly around clumps of chromatin. In the other the microtubules are found in the final spindle alignment but the chromatin is scattered through the spindle. It is suggested that the former configuration gives rise to the latter. On balance it would appear that cryptophycean spindle formation is a fairly complex phenomenon and it would appear to be difficult to explain solely on the basis of previously proposed theories of spindle formation.