The yeast protein Ccz1p is necessary for vacuolar protein trafficking and biogenesis. In a complex with Mon1p, it mediates fusion of transport intermediates with the vacuole membrane by activating the small GTPase Ypt7p. Additionally, genetic data suggest a role of Ccz1p in earlier transport steps, in the Golgi. In a search for further proteins interacting with Ccz1p, we identified the endosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptor Pep12p as an interaction partner of Ccz1p. Combining the ccz1Delta mutation with deletions of PEP12 or other genes encoding components of the endosomal fusion machinery, VPS21, VPS9 or VPS45, results in synthetic growth phenotypes. The genes MON1 and YPT7 also interact genetically with PEP12. These results suggest that the Ccz1p-Mon1p-Ypt7p complex is involved in fusion of transport vesicles to multiple target membranes in yeast cells.