Chicken beta globin locus contains four genes, two of which, rho and epsilon, are expressed from the earliest stage of primitive hematopoiesis. Here we show that the transcription of these two genes in the nucleus engages in "on/off" phases. During each "on" phase, cotranscription of rho and epsilon in cis is favored. We propose that these two chicken beta globin genes are transcribed not by competing for a transcription initiation complex, but in a cooperative way.