We isolated clones from different types of genomic Drosophila virilis libraries by screening with mRNA labelled in vitro. The DNA regions of the clones showing cross-hybridization with the RNA were sequenced. All of them contained different types of simple sequences, which most probably were solely responsible for the hybridization. We show that simple sequences and their transcription are not a phenomenon that is restricted to Drosophila. Simple sequences can be detected in all eucaryotes and at least three types (poly[d(G--T)] X poly[d(C--A)], poly[d(G--A)] X poly-[d(C--T)] and poly(dA) X poly(dT] are also widely transcribed.