Study was made of the influence of X-mutation on interallelic and intergenic recombination at the region of genes 34-38 of phage T4B. In the case of intergenic crosses in all studied recombinations of genes mutation X caused a 2-fold decrease in recombination frequency. X-mutation showed the same effect on integenic recombination in genes 36 and 38. In the case of crossing flank markers of gene 35 the frequency of recombination decreased 3.5-fold, that of gene 34--6 fold. Crosses of double am-X mutants mapped at different sites of gene 34 allowed to detect the gradient of the effect X-mutation on recombination in gene 34 in the direction from gene 33 to gene 35. The data obtained indicate a local anomaly of DNA structure of the region of genes 34 and 35 and allowed to draw a conclusion that the degree of suppressing recombination process under the influence of X-mutation depends nor on the original recombination frequency between markers to be crossed, that is on the internal structure of the given region of the genome of bacteriophage T4. Possible reasons for specific effect of X-mutations on the recombination in genes 34 and 35 of phage T4B are discussed.