Rates of copia transposition were determined directly, by cytological in situ hybridization analysis of sites of copia insertion in progeny of males from sublines of an inbred strain with different genomic copia copy numbers. Copia transposition rate was positively associated with a high order power of copia copy number. This relationship cannot be explained by copia or host mutations affecting transposition rate. The number of copia virus-like particles, necessary for copia transposition, could depend on a high order power of copia copy number. Deleterious side-effects of the transposition process may be an important force controlling copia copy number in natural populations.