How does muscle gene mutation affect the muscle structure and function of an animal? Mutant animals of the tropomyosin and troponin C genes of Caenorhabditis elegans show Pat (paralyzed, arrested elongation at twofold) phenotypes together with abnormal muscle filament assembly. We present evidence that the mutation sites of lev-11 gene was in the tropomyosin gene, tmy-1 and that of pat-10 was in the troponin C gene, tnc-1, of the worm, respectively. The lev-11 (st557) mutation occurred at the splice donor site of exon 1 and results in translation termination. Although the gene product from heterozygous (+/st557) animal was not detected, our result could be the reason for the Pat phenotype of this mutation. The lev-11(x12) mutation, isolated as an allele of levamisole resistance, occurred in exon 7 and results in amino acid substitution at 234 from Glu to Lys. This substitution give a charge change from - to + at this point which is common in three isoforms. There may be functional importance of this region for molecular interaction of the tropomyosin. Mutation site of pat-10(st575) was Asp64 to Asn and Trp153 to termination in the troponin C. The first mutation site was in the second calcium binding site and the second mutation raised the deletion of H helix in the troponin C. Both might affect the calcium binding or the retaining of the conformation for its function. Results presented here will be useful to understand the interaction site between the tropomyosin and troponin complex.