We have studied the role of three loci, quit, ovarian tumor and shut down during oocyte differentiation in Drosophila by using in situ hybridisation and double mutant analyses. Mutations in qui and otu disturb the cystocyte divisions and the oocyte determination, while mutations in shu affect the cystocyte integrity, nevertheless allowing differentiation of normal-looking egg chambers with an oocyte. In all mutants the transport of molecules towards the posterior end of the egg chamber takes place as revealed by the accumulation of Bic-D or K10 transcripts. We show that the transport is ineffective in the qui and otu mutants apparently due to the lack of a properly differentiated oocyte. In the shu mutant the transport collapses and the oocyte is lost, leading to egg chambers with 15 nurse cells. We also show that one function of qui+ is to enhance otu+ mRNA expression, suggesting that these genes control the cystocyte maturation via the same pathway.