The correlation between the plaque morphology and the presence of the insertion sequence at the phage genomes have been found for different variants of phi C43, obtained by spontaneous induction of strain S. lividans 803. Phages carrying complete insertion have Tum phenotype and they are represented in 4% of induced lysate. Phages carrying the part of insertion have Tu phenotype, their portion in induced lysate is about 26%. Phages without insertion have Stu phenotype and they are the majority of the lysate (70%). All variants phi C43, except wild-type phage, have lost the fragment of phage genome. Phage phi C43ins1, carrying the complete insertion give rise spontaneously at the frequency 10(-2) to deletion derivatives. Heteroduplex experiments show that in the deletion derivatives phi C43ins1 also in the Stu and Tu variants phi C43 one of the termini of the deletions is located within the insertion or at its end. Considerable portion of deletion variants in induced lysate phi C43 also the formation of deletion derivatives phi C43ins1 with high frequency strongly suggest that insertion sequence generates deletions similar to IS of Tn elements.