Chediak-Higashi syndrome (CHS) is characterized at the cellular level by a defect in the ability of cells to secrete lysosomes. However, the precise step affected in the secretion process is unclear. We characterized Dictyostelium discoideum cells containing a mutation in lvsB, the homolog of the human gene (LYST) involved in CHS. As observed in mammalian cells, secretion of lysosome-derived compartments was affected in lvsB mutant cells. This defect was mirrored by a decrease in the number of fusion-competent post-lysosomal compartments, which in Dictyostelium can be clearly distinguished from lysosomes. In addition, the transfer of endocytosed particles from lysosomes to post lysosomes was strongly diminished in lvsB mutant cells compared with the wild type. These results suggest that LvsB is primarily involved in transport from lysosomes to post lysosomes, and thus plays a critical role in the maturation of lysosomes into fusion-competent post-lysosomal compartments.