The two cultures under investigation were genetically engineered for the purpose of producing fuel ethanol from biomass and wastes. In this study, stability was viewed in pragmatic terms whereby the fermentation performance of the different genetic constructs was assessed solely with respect to their capacity to maintain a high efficiency of sugar-to-ethanol conversion. Two serial transfers of test tube batch cultures accounted for about 12 generations of semi-continuous growth in LB medium containing one of 4 different sugars - glucose, galactose, mannose or xylose. Both the plasmid-bearing recombinant E. coil B (ATCC 11303) pLOI297 and the chromosomally-integrated recombinant KO11 exhibited dramatic loss of ethanologenicity during growth in a selective (antibiotic-supplemented) medium with mannose as fermentation substrate. In the absence of antibiotics, both recombinants exhibited instability, with the exception of KO11 with xylose as substrate. These observations with short term cultures call into question previous claims regarding the stability of these genetic constructs.