The zebrafish embryo is transparent and can tolerate absence of blood flow because its oxygen is delivered by diffusion rather than by the cardiovascular system 1. It is therefore possible to attribute cardiac failure directly to particular genes by ruling out the possibility that it is due to a secondary effect of hypoxia. We focus here on pickwick m171 (pik m171), a recessive lethal mutation discovered in a large-scale genetic screen 2. There are three other alleles in the pik complementation group with this phenotype (pik m242, pik m740, pik m186; ref. 3) and one allele (pik mVO62H) with additional skeletal paralysis 4. The pik heart develops normally but is poorly contractile from the first beat. Aside from the edema that inevitably accompanies cardiac dysfunction, development is normal during the first three days. We show by positional cloning that the'causative'mutation is in an alternatively-spliced exon of the gene (ttn) encoding Titin. Titin is the biggest known protein and spans the half-sarcomere from Z-disc to M-line in heart and skeletal muscle 5. It has been proposed to provide a scaffold for the assembly of thick and thin filaments 6 and to provide elastic recoil engendered by stretch during diastole 7. We found that nascent myofibrils form in pik mutants, but normal sarcomeres are absent. Mutant cells transplanted to wildtype hearts remain thin and bulge outwards as individual cell aneurysms without affecting nearby wildtype cardiomyocytes, indicating that the contractile deficiency is cell-autonomous. Absence of Titin function thus results in blockage of sarcomere assembly and causes a functional disorder resembling human dilated cardiomyopathies, one form of which is described in another paper in this issue 8.