||C. elegans male tail is a structurally complex organ for locating the hermaphrodite's vulva in mating. The morphogenesis of its constituent sensory rays is a complex process tightly under genetic control. Mutations of a number of ram loci have been shown to result in abnormal ray morphology. ram (bx32) mutants have the most severe Ram phenotype whereas the rays are so swollen that individual rays cannot be distinguished . Another ram mutation, ram-2 (bx39ts) with a milder Ram phenotype, initially defined by mapping as a separate locus on LG II, has now been confirmed as allelic to ram (bx32). The bx32 allele of ram-2 was generated by EMS. I showed that ram-2 indeed encodes a cuticular collagen. It has a single nucleotide changed resulting in substitution of the glycine 126 by a glutamic acid. This mutation is at the first Gly-X-Y domain. While homozygous bx32 or bx32/Df mutants displayed the most severe Ram phenotype, it was initially interpreted to be a null allele. However, this bx32 allele presented a temperature sensitive phenotype in heterozygosity. The rays of bx32/+ animals had severely swollen ray tips at 25°C but not as serious at 20°C. Since this phenotype was not observed in +/mnDf83 (with the ram-2 locus deleted), we propose that bx32 acts as a temperature sensitive dominant negative allele instead of being a null allele. This idea was further supported by the results of interactions of ram-2 with other collagen genes and the ram-2 knockdown experiments with dsRNA. bx32 mutation alone was found to affect ray cell migration. This defect was enhanced by the ev400 allele of unc-6 netrin. In addition, the interaction between ram-2 and unc-5 or unc-6 was also observed in embryos. Double mutations could dramatically reduce the brood size, or had the brood size restored in specific alleles combination. Based on these findings, we hypothesized that RAM-2 is an extracellular matrix component, which plays an important role in both ray morphogenesis and early embryonic function, both of which may be brought about by abnormality in cellular migration.