Daughter cells with distinct fates can arise through intrinsically asymmetrical divisions. Before such divisions, factors crucial for determining cell fates become asymmetrically localized in the mother cell. In Caenorhabditis elegans, PAR proteins are required for the early asymmetrical divisions that establish embryonic polarity, and are asymmetrically localized in early blastomeres, although the mechanism of their distribution is not known. Here we report the identification in C. elegans of nonmuscle myosin II heavy chain (designated NMY-2) by means of its interaction with the PAR-1 protein, a putative Ser/Thr protein kinase. Furthermore, injections of nmy-2 antisense RNA into ovaries of adult worms cause embryonic partitioning defects and lead to mislocalization of PAR proteins. We therefore conclude the NMY-2 is required for establishing cellular polarity in C. elegans embryos.