Male zebrafish were mutagenized with N-ethyl N-nitrosourea to induce mutations in spermatogonial cells at an average specific locus rate of one in 651 mutagenized genomes, Mutations were transmitted to the F-1 generation, and 2205 F-2 families were raised. F-3 embryos from sibling crosses within the F-2 families were screened for developmental abnormalities, A total of 2337 mutagenized genomes were analyzed, and 2383 mutations resulting in abnormal embryonic and early larval phenotypes were identified. The phenotypes of 695 mutants indicated involvement of the identified loci in specific aspects of embryogenesis. These mutations were maintained for further characterization and were classified into categories according to their phenotypes, The analyses and genetic complementation of mutations from several categories are reported in separate manuscripts, Mutations affecting pigmentation, motility, muscle and body shape have not been extensively analyzed and are listed here, A total of 331 mutations were tested for allelism within their respective categories, This defined 220 genetic loci with on average 1.5 alleles per locus. For about two-thirds of all loci only one allele was isolated, Therefore it is not possible to give a reliable estimate on the degree of saturation reached in our screen; however, the number of genes that can mutate to visible embryonic and early larval phenotypes in zebrafish is expected to be several-fold larger than the one for which we have observed mutant alleles during the screen. This screen demonstrates that mutations affecting a variety of developmental processes can be efficiently recovered from zebrafish.