Outlines of the genetic circuitry regulating male gametogenesis in Drosophila have begun to appear. Cessation of mitotic proliferation and onset of the meiotic program is regulated by the bam and bgcn genes acting within male germ cells and a TGF-beta class signaling cascade in surrounding somatic cells. Onset of spermatid differentiation is regulated by a stage- and tissue-specific transcriptional program controlled by the aly, can, mia and sa genes. A cross-regulatory mechanism might act, in part by controlling expression of the twine cell cycle phosphatase, to delay the G2/M transition of meiosis I until genes required for spermatid differentiation have been transcribed.