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The lysosome is considered the terminal point in the endocytic pathway, as molecules are delivered to the lysosome for degradation (for review see (1)). Fusion of endosomes with lysosomes leads to the intermixing of vesicular contents with lysosomal hydrolases. The addition of vesicular contents and membrane would be expected to increase lysosome size. The steady state median size of lysosomes is constant, however, indicating that lysosomal surface area is regulated by lysosomal membrane efflux. It has been suggested that endocytic membrane influx is transient and that once endosomal contents are transferred to lysosomes, endosomes fission off, a process referred to as “kiss-and-run” (2). Constant fusion and fission of membrane along the endocytic pathway results in remodeling of organelles, which may eventually become lysosomes, a process termed maturation (3). Lysosomes are also capable of homotypic fusion (4–13) yet the number of lysosomes is constant. Therefore, lysosomes must be capable of fission or some other form of membrane recycling, although the machinery required for lysosome fission has not been identified. Malregulation of fusion or fission will result in alterations in lysosome size. This is a hallmark of Chediak Higashi syndrome (CHS), a rare autosomal recessive disorder characterized by enlarged lysosomes or lysosome-related organelles present within all cells (14–17). The mutated gene responsible for CHS (LYST) and the orthologous mouse disorder beige (Lyst) were identified almost 15 years ago (18, 19). The identification of Chs1/Lyst defined a family of proteins termed the BEige And CHediak (BEACH) family (15, 20). The BEACH family protein LvsA in Dictyostelium discoideum regulates the size of the contractile vacuole and is involved in cytokinesis (21–23). Neurobeachin has been characterized to be involved in central synapse formation (24). FAN, the smallest member of the BEACH family, is thought to be an adapter protein linking TNFa signaling to neutral sphingomyelinase. Bph1, the only Saccharomyces cerevisiae homologue is involved in vesicle trafficking, but loss of Bph1 does not affect vacuole morphology (25). The functions of many members of the BEACH family are still unclear. Previously, we suggested that loss of Chs1/Lyst resulted in decreased lysosomal fission based upon the observation that overexpression of Lyst resulted in smaller than wild type lysosomes (26). Fusion of wild type and beigej cells led to complementation of the large lysosome size (27). We noted, however, that complementation of large beigej lysosomes required their fusion with wild type lysosomes, suggesting that the wild type lysosome provided a factor promoting decreased lysosome size. Studies on the D. discoideum homologue LvsB suggested that LvsB was either a negative regulator of lysosome fusion (28, 29) or a positive regulator of “post-lysosome” fission (30). In this study we examine whether the loss of Chs1/Lyst changes the rate of lysosome fusion or lysosome fission by measuring the rate of reformation of lysosomes following treatments that increase or decrease lysosome size. We show that the loss of Chs1/Lyst does not affect lysosome fusion rates but affects the rate of lysosome fission and that it is the defect in fission that gives rise to enlarged lysosomes associated with the loss of Chs1/Lyst.