The zebrafish pronephric kidney provides a simplified model of nephron development and epithelial cell differentiation which is amenable to genetic analysis. The pronephros consists of two nephrons with fused glomeruli and paired pronephric tubules and ducts. Nephron formation occurs after the differentiation of the pronephric duct with both the glomeruli and tubules being derived from a nephron primordium. Fluorescent dextran injection experiments demonstrate that vascularization of the zebrafish pronephros and the onset of glomerular filtration occurs between 40 and 48 hpf. We isolated fifteen recessive mutations that affect development of the pronephros. All have visible cysts in place of the pronephric tubule at 2-2.5 days of development. Mutants were grouped in three classes: (1) a group of twelve mutants with defects in body axis curvature and manifesting the most rapid and severe cyst formation involving the glomerulus, tubule and duct, (2) the fleer mutation with distended glomerular capillary loops and cystic tubules, and (3) the mutation pao pao tang with a normal glomerulus and cysts limited to the pronephric tubules. double bubble was analyzed as a representative of mutations that perturb the entire length of the pronephros and body axis curvature. Cyst formation begins in the glomerulus at 40 hpf at the time when glomerular filtration is established suggesting a defect associated with the onset of pronephric function. Basolateral membrane protein targeting in the pronephric duct epithelial cells is also severely affected, suggesting a failure in terminal epithelial cell differentiation and alterations in electrolyte transport. These studies reveal the similarity of normal pronephric development to kidney organogenesis in all vertebrates and allow for a genetic dissection of genes needed to establish the earliest renal function.