We were interested in identifying genes in the dipteran flies Calliphora erythrocephala and Drosophila melanogaster with a role in oogenesis and early embryogenesis. A biochemical screen was used to complement the extensive genetic screens that have been performed to identify such genes in Drosophila. Radio-labelled cDNA probes were made using poly(A)\sp+ RNA preparations from staged Calliphora ovaries and embryos. These probes were used to isolate clones which were strongly expressed during oogenesis but not during embryogenesis. Four Calliphora genes which are absolutely "oogenesis-specific" in their expression pattern, as defined by our screening protocol, were identified. These are called A10B, B8I, C7F and GG7K. Three of these clones are expressed in the somatically derived follicle cells of the ovary and have been identified as being homologous to the Drosophila yolk protein 1 (A10B and B8I) and a vitelline membrane protein (GG7K). Interestingly, the yolk protein homologs are expressed in a specialized subset of follicle cells known as the border cells in Calliphora. The fourth gene, (C7F) is expressed in the nurse cells, the transcripts are translocated to the oocyte proper and maintained throughout the first four hours of embryogenesis. C7F is also expressed in late pupae and adult male flies. A number of Calliphora genes were identified which, although not oogenesis-specific, were more strongly expressed in the oocyte than the embryo. These were classified as "oogenesis-differentials". C7F and the Calliphora oogenesis-differential genes were used to screen Drosophila cDNA and genomic libraries for homologs. Some characterization of these Drosophila homologs is described here. ME31B, a maternally expressed Drosophila gene from the 31B region of the second chromosome was isolated by directly screening Drosophila libraries with Calliphora cDNA probes. ME31B is expressed throughout oogenesis and the transcript is maintained in the mature egg until four hours after fertilization. The ME31B transcript is evenly distributed throughout the oocyte and egg. A 1.5 kb cDNA for ME31B was completely sequenced. Comparison of the coding sequence with a protein data bank allowed us to show that ME31B is a member of a family of NTP-dependent helicases. The possible mutant phenotype of ME31B is discussed.