Mutations in gene 49 of bacteriophage T4 were suppressed by a second mutation in gene X. Mapping studies located gene X between genes 41 and 42. Complementation results indicated that mutations in FdsA gene (a suppressor of gene 49 mutants) were in gene X. The intracellular pseudorevertant DNA was examined for unusual properties which could explain its successful encapsidation. After the in vivo inactivation of a temperature-sensitive gene 32 (DNA unwinding) protein, the intracellular pseudorevertant DNA was converted into DNA pieces of approximately genome size. A similar conversion was observed after in vitro digestion of pseudorevertant DNA with single-strand-specific S1 endonuclease. Appreciable quantities of oligomeric intermediates were not produced during this conversion process. These data indicate that pseudorevertant DNA contains sizable single-stranded gaps and has a conformation similar to that of wild-type DNA. The results further suggest that the suppression of gene 49 mutant abnormal DNA phenotype and the encapsidation defect by a second mutation in gene X is associated with the formation of sizable single-stranded gaps. These studies raise the possibility that single-stranded gaps may be involved directly in the DNA encapsidation process, or may act indirectly as a consequence of their effect on the organization of intracellular DNA.