The Arp2/3 complex is a highly conserved cytoskeletal component that has been implicated in the nucleation of actin filament assembly [1]. Purified Arp2/3 complex has a low intrinsic actin nucleation activity, leading to the hypothesis that an unidentified cellular activator is required for the function of this complex [2 3]. We showed previously that mutations in the Arp2/3 complex and in Bee1p/Las17p, a member of the Wiskott–Aldrich syndrome protein(WASP) family, lead to a loss of cortical actin structures (patches) in yeast [4 5]. Bee1p has also been identified as an essential nucleation factor in the reconstitution of actin patches in vitro[6]. Recently, it was reported that WASP-like proteins might interact directly with the Arp2/3 complex through a conserved carboxy-terminal domain [7]. Here, we have shown that Bee1p and the Arp2/3 complex co-immunoprecipitate when expressed at endogenous levels, and that this interaction requires both the Arc15p and Arc19p subunits of the Arp2/3 complex. Furthermore, the carboxy-terminal domain of Bee1p greatly stimulated the nucleation activity of purified Arp2/3 complex in vitro, suggesting a direct role for WASP-family proteins in the activation of the Arp2/3 complex. Interestingly, deletion of the carboxy-terminal domain of Bee1p neither abolished the localization of the Arp2/3 complex, as had been suggested, nor resulted in a severe defect in cortical actin assembly. These results indicate that the function of Bee1p is not mediated entirely through its interaction with the Arp2/3 complex, and that factors redundant with Bee1p might exist to activate the nucleation activity of the Arp2/3 complex.