Abstract We isolated twelve suppressor sensitive ( sus) mutants from the RNA phage SP which belongs to group IV and classified them into three cistrons (genes 1, 2, and 3) by complementation tests. Using representative mutants of each cistron, synthesis of infectious RNA, phage antigen (serum blocking power) and defective particles under nonpermissive conditions were examined. Gene 1 mutants had a defect in maturation protein synthesis, gene 2 mutants in coat protein synthesis, and gene 3 mutants in RNA synthesis, respectively. Thus, group IV RNA phage SP has (at least) three genes corresponding to RNA replicase, coat protein, and maturation protein, as already shown in group I (f2, R17) and group III (Qβ) RNA phages. From intergroup complementation tests between several sus mutants of groups I, III, and IV, Qβ(III) had a fairly close relationship to SP(IV), while f2(I) had no such close relationships to Qβ(III) or SP(IV). These are consistent with the results of grouping by several biological and physicochemical criteria and support the idea that groups I and II may be assembled into one group and groups III and IV into another.