When the dmd gene of bacteriophage T4 is mutated, many T4 late genes are post-transcriptionally silenced because of rapid mRNA degradation. Here we show that the host hfq gene is involved in the rapid mRNA degradation in a dmd mutant. A disruption of the hfq gene caused weak but significant effects on the stability of late-gene mRNA, the late-gene expression and the growth of a dmd mutant. By probing with the soc gene, we found that disruption of the hfq gene impaired the translation-independent mRNA degradation, one of two mechanisms promoting rapid mRNA degradation. We also showed that purified Hfq protein bound stoichiometrically to soc RNA. These results strongly suggest that the hfq gene has a stimulatory role in dmd mutant-specific mRNA degradation.