During vertebrate development, neural crest cells are thought to pattern many aspects of head organization, including the segmented skeleton and musculature of the jaw and gills. Here we describe mutations at the gene chinless, chn, that disrupt the skeletal fates of neural crest cells in the head of the zebrafish and their interactions with muscle precursors. chn mutants lack neural-crest-derived cartilage and mesoderm- derived muscles in all seven pharyngeal arches. Fate mapping and gene expression studies demonstrate the presence of both undifferentiated cartilage and muscle precursors in mutants. However, chn blocks differentiation directly in neural crest, and not in mesoderm, as revealed by mosaic analyses. Neural crest cells taken from wild-type donor embryos can form cartilage when transplanted into chn mutant hosts and rescue some of the patterning defects of mutant pharyngeal arches. In these cases, cartilage only forms if neural crest is transplanted at least one hour before its migration, suggesting that interactions occur transiently in early jaw precursors. In contrast, transplanted cells in paraxial mesoderm behave according to the host genotype; mutant cells form jaw muscles in a wild-type environment. These results suggest that chn is required for the development of pharyngeal cartilages from cranial neural crest cells and subsequent crest signals that pattern mesodermally derived myocytes.