The promoter region of the developmentally regulated Actin 6 gene of Dictyostelium has been dissected by a series of deletions. Functional analysis of the deletions in Dictyostelium transformants revealed two short regulatory sequences: a positive upstream element (PUE) between -599 and -572 which increases transcription by a factor of 10 but does not affect the developmental pattern of expression and an upstream activator sequence (UAS) between -249 and -215 which is essential for transcription and proper developmental regulation. The UAS partially coincides with a conserved sequence with dyad symmetry found upstream of several Dictyostelium actin genes (Romans and Firtel, 1985a).