Different characteristics of flies relating to sigma virus allow us to class the following drosophila genotypes according to their permissivity for the virus strains which are sensitive to the Pp allele: (formula: see text). It is concluded 1) that the two alleles Po and Pp of ref(2)P gene are active and 2) that the viral protein which interact with the product of ref(2)P is effective, or effectively transformed, without interaction with the product of ref(2)P. The delayed appearance of CO2 sensitivity symptom in flies which are issued from stabilized maternal lines, while they are immune to a superinfection non Pp sensitive virus, leads us to believe that ref(2)P is active not only on a function necessary to viral genome replication, as assumed by preceding workers, but also on a function necessary to maturation for the viral strain which was used.