The insertion site number of the retrotransposable element 412 was analyzed in natural populations of Drosophila simulans of worldwide origin by in situ hybridization. We observe a gradient in copy number ranging from as high as 23 in Europe to 1-10 in South Africa, while populations in Madagascar and the Indian Islands, which are the cradle of D. simulans, have only 3-7 copies. We find very different copy numbers in some local populations of Australia and the Pacific Islands (with around 60 copies in 1 sample and only 5 in another), suggesting spontaneous transposition bursts in local populations. Such bursts occurring now and then in local natural populations followed by fly migration could lead to the progressive invasion of the entire species by the transposable element mobilized, explaining the gradient in 412 copy number between northern and southern hemispheres.