In a large scale screen for mutants that affect the early development of the zebrafish, 109 mutants were found that cause defects in the formation of the jaw and the more posterior pharyngeal arches. Here we present the phenotypic description and results of the complementation analysis of mutants belonging to two major classes: (1) mutants with defects in the mandibular and hyoid arches and (2) mutants with defects in cartilage differentiation and growth in all arches. Mutations in four of the genes identified during the screen show specific defects in the first two arches and leave the more posterior pharyngeal arches largely unaffected (schmerle, sucker, hoover and sturgeon). In these mutants ventral components of the mandibular and hyoid arches are reduced (Meckel's cartilage and ceratohyal cartilage) whereas dorsal structures (palatoquadrate and hyosymplectic cartilages) are of normal size or enlarged. Thus, mutations in single genes cause defects in the formation of first and second arch structures but also differentially affect development of the dorsal and ventral structures within one arch. In 27 mutants that define at least 8 genes, the differentiation of cartilage and growth is affected. In hammerhead mutants particularly the mesodermally derived cartilages are reduced, whereas jellyfish mutant larvae are characterized by a severe reduction of all cartilaginous elements, leaving only two pieces in the position of the ceratohyal cartilages. In all other mutant larvae all skeletal elements are present, but consist of smaller and disorganized chondrocytes. These mutants also exhibit shortened heads and reduced pectoral fins. In homozygous knorrig embryos, tumor-like outgrowths of chondrocytes occur along the edges of all cartilaginous elements. The mutants presented here may be valuable tools for elucidating the genetic mechanisms that underlie the development of the mandibular and the hyoid arches, as well as the process of cartilage differentiation.