P element excision generates a DNA double-strand break at the transposon donor site. Genetic studies have demonstrated a strong bias toward repair of P element-induced DNA breaks by homologous recombination with the sister chromatid, suggesting that P element excision occurs after DNA replication, in G 2 of the cell cycle. We developed methods to arrest Drosophila tissue culture cells and assay P element excision in either G 1 - or G 2 -arrested cells. Dacapo or tribbles transgene expression arrests cells in either G 1 or G 2 , respectively. RNA-mediated gene interference (RNAi) directed against cyclin E or cyclin A arrests cells in G 1 or G 2 , respectively. P element excision occurs efficiently in both G 1 - and G 2 -arrested cells, suggesting that cell cycle regulation of P element transposase does not occur in our somatic cell system. DNA double-strand break repair occurs by two predominant mechanisms: homologous recombination (HR) and non-homologous end joining (NHEJ). HR is thought to be restricted to the post-replicative, G 2 , phase of the cell cycle, while NHEJ may occur throughout the cell cycle. Our results indicate that NHEJ repair of an extrachromasomal plasmid substrate occurs at least as efficiently in G 2 -arrested cells as in asynchronous cells or in G 1 -arrested cells.