We have used monoclonal antibodies to perform confocal light microscopic immunolocalization of KRP(85/95), a heterotrimeric plus-end-directed microtubule motor protein, in dividing cells of sea urchin embryos. Embryos were stained during the first division cycle, and dissociated blastomeres were stained at the 32- to 64-cell stages. Double labeling of the dividing cells with anti-tubulin and anti-KRP(85/95) showed a clear concentration of the motor protein in the mitotic apparatus; KRP(85/95) appeared to associate with pericentriolar regions during prophase, with kinetochore-to-pole microtubules during metaphase, and, in a striking fashion, with the spindle interzone during anaphase. KRP(85/95) began to accumulate in the interzone immediately following chromosome separation and the area of concentration expanded with the lengthening of the interzonal region during anaphase. During telophase KRP(85/95) appeared to disperse with the establishment of the cleavage furrow and did not concentrate in the midbody. KRP(85/95) staining in the mitotic apparatus was punctate and detergent-sensitive, suggesting an association with membranous vesicles, but unlike kinesin, KRP(85/95) did not appear to codistribute with calsequestrin-containing endoplasmic reticulum. Finally, KRP(85/95) appears to be present in dividing blastomeres up to at least the blastula stage, but, unlike kinesin, it is not expressed in terminally differentiated, nonmitotic coelomocytes of the adult animal. These results suggest that the expression and targeting of KRP(85/95) and kinesin differ and that KRP(85/95) may play a role in vesicle transport during embryonic cell division.