The tsetse fly (Dipteria: Glossinidae) relies on its obligate endosymbiont, Wigglesworthia glossinidia, for fertility and nutrition. Wigglesworthia is related to Escherichia coli, but full characterization has been limited as it cannot be cultivated in vitro. In the June 19 Proceedings of the National Academy of Sciences, Akman and Aksoy describe how they used E. coli gene arrays to study the Wigglesworthia genome (Proc Natl Acad Sci USA 2001, 98:7546-7551). They painstakingly isolated bacterial DNA by microscopic dissection of bacterial tissue from 200 tsetse guts. Using contour-clamped homogeneous electric field (CHEF) electrophoresis they determined that the Wigglesworthia genome is less than 770 kilobases (about one sixth the size of E. coli). They hybridized Wigglesworthia DNA to E. coli microarrays to identify 650 orthologous genes, which are predicted to represent about 85% of its genome. Analysis of Wigglesworthia cDNA using the E. coli arrays detected 61 genes, presumably representing the most abundantly expressed subset of genes. Over half of these have known functions in E. coli, including DNA replication, transcription, ammonium transport and vitamin biosynthesis. On the basis of the genes identified, Akman and Aksoy predict that Wigglesworthia is a facultative anaerobe that feeds on ammonia as a nitrogen source. This study shows how heterologous gene arrays can shed light on micro-organisms whose genomes have not been sequenced.