Two types of collagens have been identified in Caenorhabditis elegans corresponding to two types of extracellular matrix, the cuticle and basement membranes. Cuticle collagens are encoded by a developmentally regulated family of approximately 100 genes. Mutations in cuticle collagens can produce animals that are longer or shorter than normal and/or that are helically twisted. Mutations in different collagens can cause different morphological abnormalities, as can different mutations in the same collagen. Genetic interactions between collagen genes have been described and may identify collagens that interact to form the cuticle. Two basement membrane (type IV) collagen genes have been identified in C. elegans. They encode proteins similar in structure to vertebrate type IV collagen. One of the genes produces two alternatively spliced forms, one predominantly expressed in embryos and the other in larvae and adults, suggesting that embryonic basement membranes may have unique properties. Most mutations in the type IV genes cause embryonic lethality, indicating that normal basement membranes are required for embryogenesis. Temperature-sensitive mutations have been used to show that type IV collagen function is also required for larval development and adult fertility.