Cell divisions that create daughter cells of different sizes are crucial for the generation of cell diversity during animal development. In such asymmetric divisions, the mitotic spindle must be asymmetrically positioned at the end of anaphase. The mechanisms by which cell polarity translates to asymmetric spindle positioning remain unclear. Here we examine the nature of the forces governing asymmetric spindle positioning in the single-cell-stage Caenorhabditis elegans embryo. To reveal the forces that act on each spindle pole, we removed the central spindle in living embryos either physically with an ultraviolet laser microbeam, or genetically by RNA-mediated interference of a kinesin. We show that pulling forces external to the spindle act on the two spindle poles. A stronger net force acts on the posterior pole, thereby explaining the overall posterior displacement seen in wild-type embryos. We also show that the net force acting on each spindle pole is under control of the par genes that are required for cell polarity along the anterior-posterior embryonic axis. Finally, we discuss simple mathematical models that describe the main features of spindle pole behaviour. Our work suggests a mechanism for generating asymmetry in spindle positioning by varying the net pulling force that acts on each spindle pole, thus allowing for the generation of daughter cells with different sizes.