Osteogenesis imperfecta type I is a mild, dominantly inherited, connective tissue disorder characterized by bone fragility. Mutations in type I collagen account for all known cases. In Mov-13 mice, integration of a murine retrovirus within the first intron of the alpha 1(I) collagen gene results in a null allele blocked at the level of transcription. This study demonstrates that mutant mice heterozygous for the null allele are a model of osteogenesis imperfecta type I. A defect in type I collagen production is associated with dominant-acting morphological and functional defects in mineralized and nonmineralized connective tissue and with progressive hearing loss. The model provides an opportunity to investigate the effect of a reduced amount of type I collagen on the structure and integrity of extracellular matrix. It also may represent a system in which therapeutic strategies to strengthen connective tissue can be developed.