Abstract: GTPases of the Rab family cycle between an inactive (GDP-bound) and active (GTP-bound) conformation. The active form of the Rab regulates a variety of cellular functions via multiple effectors. Guanine nucleotide exchange factors (GEFs) activate Rabs by accelerating the exchange of GDP for GTP, while GTPase activating proteins (GAPs) inactivate Rabs by stimulating the hydrolysis of GTP. The GTPase Ypt1p is required for ER-Golgi and intra-Golgi traffic in the yeast Saccharomyces cerevisiae. Recent findings, however, have shown that a Ypt1p GEF, GAP and effector are all required for traffic from the early endosome to the Golgi. Here we describe a screen for ypt1 mutants that block traffic from the early endosome to the late Golgi, but not general secretion. This screen has led to the identification of a collection of recessive and dominant mutants that block traffic from the early endosome. While it has long been known that Ypt1p regulates the flow of biosynthetic traffic into the cis side of the Golgi, these findings have established a role for Ypt1p in the regulation of early endosome-Golgi traffic. We propose that Ypt1p regulates the flow of traffic into the cis and trans side of the Golgi via multiple effectors.