The zebrafish (Danio rerio) is a useful model for understanding normal and cancer stem cells, but analysis has been limited to embryogenesis due to the opacity of the adult fish. To address this, scientists have recently created a transparent adult zebrafish in which either hematopoietic stem/progenitor cells or tumor cells have been transplanted. In a hematopoiesis radiation recovery assay, transplantation of GFP-labeled marrow cells allowed for striking in vivo visual assessment of engraftment from 2 hours-5 weeks post transplant. Using FACS analysis, both transparent and wild-type fish had equal engraftment, but this could only be visualized in the transparent recipient. In a tumor engraftment model, transplantation of RAS-melanoma cells allowed for visualization of tumor engraftment, proliferation and distant metastases in as little as 5 days, which is not seen in wild-type recipients until 3-4 weeks. This transparent adult zebrafish serves as the ideal combination of both sensitivity and resolution for in vivo stem cell analyses.