We report that cells of a Drosophila embryonic cell line (Kc167 cells) can be readily and stably transformed by transposition of P elements from exogenous DNA. Cells are transfected with plasmids carrying methotrexate- or α-amanitin-resistance markers expressed from constitutive promoters and co-transfected with a gene encoding a somatically active transposase. Transient expression of the transposase leads to efficient production of transformed, resistant cells. We describe conditions under which most resistant clones are healthy and harbor a small number (1-50) of transposons and few (≤5%) retain plasmid sequences derived from illegitimate recombination. Using conditions like these it should prove possible to construct enhancer trap and/or gene libraries using Drosophila cells.