A follow-up over 83 generations has been carried out, by the Southern blotting 
technique, of a Drosophila stock which is unstable in the location of Bari 1 
elements. The persistent intrastock polymorphism detected is largely amenable to 
insertion/excision equilibria at 36 genomic sites that form a gradient in 
occupancy. In a closely related stock, Bari 1 elements are stable and exhibit a 
substantially different genomic distribution. These results suggest that in 
Drosophila preferential insertion sites may be defined with the contribution of 
host factors, although alternative interpretations are also possible. The 
relevance to the mechanism(s) that contains the potentially deleterious effects 
of transposition is discussed.