We describe a method of genome-wide analysis of quantitative growth phenotypes using insertional mutagenesis and DNA microarrays. We applied the method to assess the fitness contributions of Escherichia coli gene domains under specific growth conditions. A transposon library was subjected to competitive growth selection in Luria-Bertani (LB) and in glucose minimal media. Transposon-containing genomic DNA fragments from the selected libraries were compared with the initial unselected transposon insertion library on DNA microarrays to identify insertions that affect fitness. Genes involved in the biosynthesis of nutrients not provided in the growth medium were found to be significantly enriched in the set of genes containing negatively selected insertions. The data also identify fitness contributions of several uncharacterized genes, including putative transcriptional regulators and enzymes. The applicability of this high-resolution array selection in other species is discussed.