DAUGHTER cells with distinct fates can arise through intrinsically asymmetrical divisions¹. Before such divisions, factors crucial for determining cell fates become asymmetrically localized in the mother cell^2,3. In Caenorhabditis elegans, PAR proteins are required for the early asymmetrical divisions that establish embryonic polarity^4–8, and are asymmetrically localized in early blastomeres^9,10, although the mechanism of their distribution is not known. Here we report the identification in C. elegans of a non-muscle 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 that NMY-2 is required for establishing cellular polarity in C. elegans embryos.