The allele Pp of gene ref(2)P of Drosophila melanogaster is known to inhibit the multiplication of certain strains of Sigma virus. From a viral strain for which the allele PP of ref(2)P is nonpermissive, host adapted (haP) mutants were selected after mutagenesis in vivo. Mutagenized virus was extracted from infected flies fed with 5-fluorouracil. The mutagenized virus was used to inoculate restrictive flies to screen haP mutants. The surviving viruses were studied in permissive and restrictive hosts. In a first test the capability to induce CO2 symptom in permissive and in restrictive flies was studied for each surviving clone. In a second test the efficiency to initiate infection in permissive and in restrictive flies was compared to the wild-type efficiency for each putative adapted clone. From 111 surviving viruses, 23 haP mutants were thus isolated. The proportion of temperature-sensitive mutants was higher among haP mutants than among non-haP brother clones. Five ts-haP mutants appeared more temperature sensitive in the restrictive host than in the permissive one. In the five cases, the viral functions affected by the mutation were early, i.e., occurring within 24 hr of the infection of a cell at 20 degrees and related to the replication of the viral genome. These results suggest that viral proteins required for early viral functions are implicated in the interaction with the PP allele of the ref(2)P host gene.