Mutations in ilvO, the operator site for operon A of the ilv region of E. coli K-12, are reported to cause a cis dominant derepression of the operon A gene products and to map between ilvC and ilvA. Studies reported here demonstrate that the F25 episome which does not carry the region between ilvC and ilvAbut only ilvE, ilvD and a part of ilvA, can transfer ilvO genetic material. Also, genes of operon A on the F25 episome respond normally to a derepression signal. It is proposed that ilvO is on the F25 episome. Five-point crosses demonstrate that mutations in ilvOmap nearer to ilvE than to ilvA. Considering this and other evidence, it is proposed that the gene order in the ilv region is CADEO, not COADE, with transcription and translation in operon A being from ilvE to ilvA not ilvA to ilvE. It is confirmed that mutations in ilvO cause a cis dominant derepression of the operon A gene products but it is also noted that mutations in ilvO lead to the appearance of an AHAS activity more resistant to inhibition by valine than that of the present O+ strain.