Proper cell morphogenesis is dependent on the establishment and expression of cellular polarity. In the fucoid zygote, cell shape is critical for establishing the developmental pattern of the adult, and is achieved by guiding insertion of new membrane and wall to the rhizoid tip. Selection and growth of the appropriate tip site are accompanied by formation of dynamic actin arrays associated with the actin-nucleating Arp2/3 complex. In eukaryotes, a major pathway for activation of the Arp2/3 complex is via the Rho family GTPase, Rac1, which stimulates the Scar/WAVE complex. To determine whether Rac1 controls actin nucleation in Silvetia compressa (J. Agardh) E. Serrao, T. O. Cho, S. M. Boo et Brawley, we tested the effects of the Rac1-specific inhibitory compound, NSC23766, on actin dependent processes and on actin arrays. We found that NSC23766 disrupted polar secretion of adhesive, polarization of endomembranes, and tip-focused growth in the rhizoid. Similarly, NSC23766 altered actin and Arp2 localization in the growing rhizoid. In contrast, NSC23766 had no effect on selection of the growth site or on cytokinesis. These data suggest that Rac1 participates in nucleation of specific actin arrays in the developing zygote.