In studying molecular mechanisms of the formation of transducing bacteriophages, we have elucidated the primary structure of the phage-bacterial DNA junction which resulted from the abnormal excision of the λplac10 phage. The process is structurally similar to the excision of the λplac5 phage and involves, in both cases, highly homological DNA stretches approximately 20 bp long, one of them being a part of the Z-Y spacer of the lac operon and possessing a developed secondary structure. The conception of regioselective recombination as a type of illegitimate recombinational process with a certain degree of site-specificity is suggested. © 1989.