In Streptomyces ambofaciens a genetic instability generates a high degree of polymorphism consisting of four main phenotypes: pigmented colonies Pig qualified as WT phenotype , pigment-defective colonies, pigmented colonies with pigment-defective sector and pigmented colonies with pigment-defective papillae. Molecular analysis of Pig and Pig col sec. pigment-defective mutant derived from a colony and a sector, respectively produced by genetic instability and isolated in five Pig subclones progenies revealed a new aspect of polymorphism in S. ambofaciens ATCC23877. Frequencies of Pig and Pig mutants deleted at the chromosome ends varied from one WT progeny to another. Two main types of col sec deleted mutants were observed: deleted for one or both chromosomal extremities. The relative proportion of these two categories differed according to the WT progeny. These results argue for heterogeneity of the WT clones, i.e., Pig colonies, originated from S. ambofaciens ATCC23877.