Three non-homogeneous patterns of cytoplasmic actin distribution have been demonstrated in pairs of 8-cell blastomeres. Newly formed blastomeres showed an actin distribution associated with the remnant of the previous mitotic spindle. Subsequently blastomeres showed a zonal clearing of actin from regions of intercellular contact, the extent of the clearing increasing with the extent of contact. A polarized distribution of actin was evident from the early to mid 8-cell stage and coincided with the movement of nuclei towards the point of intercellular contact. The detection of polar actin preceded by 2−4 h the detection of a surface polarity as assessed by the FITC-Con A binding pattern and the distribution of cortical microvillous actin. However, once a surface pole of microvilli had formed, it persisted under conditions which led to loss of polar cytoplasmic actin. Incubation in cytochalasin D (CCD) resulted in a dispersed homogeneous pattern of actin distribution but did not prevent the formation of surface poles as assessed both by the Con A binding pattern and detection of polar microvilli. However, the poles formed were less clearly defined and the density and length of microvilli within them was variable. Moreover, when CCD was added early during the 8-cell stage the position of the poles was frequently not on an axis perpendicular to the point of intercellular contact. Cytochalasin D also affected the movement of the nucleus that occurs during the process of polarization. On the basis of these experiments, we conclude that actin is likely to be involved in the events of polarization, but that its precise role remains to be determined.