Ether resistant strain (Eth-29) and one of the sensitive strains (bw; st; svn) which were assessed by mortalities, were used to determine median effective doses (EDbo) of halothane and chloroform. It was demonstrated that the Eth-29 strain was resistant and the bw; st; svn strain was sensitive to both of halothane and chloroform anesthesia. Reciprocal crosses between the two strains suggested that the resistance to halothane anesthesia was a sex-linked recessive trait and that the resistance to chloroform anesthesia was an autosomal incompletely dominant or a polygenic trait. Maternal effects may not be negligible in both of the resistant traits.Ether resistant (Eth) strain was developed from one female fly which was hard to be anesthetized by ether. On the other hand, more than a hundred stock strains were tested for their ether sensitivity and more sensitive strains to ether were used as the ether sensitive strains. We have carried out genetic studies of the resistance to volatile anesthetics by using those resistant and sensitive strains. It was revealed that the Eth strain was resistant and that the bw; st; svn strain was sensitive to ether (Ogaki et al. 1967), chloroform (Gamo et al. 1979) and halothane (Gamo et al. in preparation). Thus, the Eth strain shows a cross-resistance to ether, chrolof orm and halothane at strain level. However, the resistances to the three anesthetic agents may be controlled by different genes, because the locus of major gene(s) to the ether resistance is on mapunit 61± of the third chromosome and the minor genes are on the first (X) and fourth chromosomes (Ogaki et al. 1967), and the major gene(s) to the chloroform resistance is located on the X chromosome and the minor gene(s) is on the second chromosome (Gamo et al. 1979), although both of the ether and chloroform resistances are completely 1) The work was cation, Japan.supported in part by a Grant-in-Aid (No. 440009) from the Ministry of Edu-