Laboratory strains of Drosophila melanogaster were screened for spatial variations in adult midgut alpha-amylase (1,4-alpha-D-glucan glucanohydrolase, EC 3.2.1.1) expression. No strain-specific differences were found anteriorly, but three patterns of activity were discerned in the posterior midgut: A, activity throughout most of the region; B, activity in the anterior part of the region; and C, little or no activity. Alleles of a control gene, map, are responsible for this tissue-specific regulation of activity; e.g., mapA homozygotes produce the A pattern and mapC homozygotes the C pattern. The map locus was placed at 2--80 +/- on the genetic map of chromosome 2R, about two crossover units distal to the Amy structural gene region for alpha-amylase. Electrophoretic studies showed that mapA is trans acting in mapA/mapC flies, allowing expression of amylase isozymes coded for by genes on the opposite chromosome. The map gene behaves as a temporal gene that is clearly separable from the tightly linked, duplicated Amy structural genes.