The structural gene for the Escherichia coli enzyme amylomaltase, malQ, is the second gene in the malPQ operon. The nucleotide sequence of malQ shows that the gene encodes an Mr 78360 protein close to the experimentally determined Mr of purified amylomaltase (72000-74000). The malQ initiation codon was identified by sequence analysis of clustered deletions around the 5' end of the gene. One of these deletions removed the first 5 bases from the malQ coding sequence. Strains carrying a plasmid with this truncated malQ gene under lacZ promoter control and out-of-frame with the first four codons of lacZ were Mal-. The Mal+ phenotype could be restored by inserting small, random fragments of E. coli chromosomal DNA into the unique EcoRI site. Nucleotide sequencing showed that the inserts either joined the lacZ and malQ sequences in frame, or contained a new translation start signal and coding sequence in frame with malQ. These results indicate that amylomaltase could be useful as a reporter protein in gene fusion studies.