The ultraspiracle (usp) locus encodes a member of the nuclear hormone receptor superfamily in Drosophila melanogaster that mediate the hormone action by heteromerization with ecdysone receptor (EcR). previous studies have shown that usp protein is localized in the nucleus with ubiquitous distribution throughout development, suggesting the pleiotropy of usp gene function during development. To investigate the usp gene function in vivo, we have isolated three recessive lethal usp mutations through EMs mutagenesis. The lethal phases of three usp mutants(usp/Y male derived from heterozygous usp/+ mothers) were 1st or 2nd larval stage, and aneuploids carrying two usp mutant alleles and a putative variegating usp+ allele often developed deformities of the adult wing disc(cleft thorax phenotype) and third leg disc. the molecular sequence changes associated with a subset of three mutations were determined. Among three alleles found in this study, two alleles (usp554, usp564) showed an identical base changes (130 Arg to Cys) as shown in usp. The other recessive lethal usp mutation (usp599) showed a substitution of amino acid (460 Arg to His) in hormone binding domain.