The present invention provides an advance in phage display technology by permitting the uncoupling of the propagation of phages containing inserted sequences encoding heterologous polypeptides from the expression of said polypeptides. The invention provides phage constructs and methods for their use to permit phage coat protein expression, and thus phage propagation, in the absence of display of heterologous polypeptides, which may be expressed as a fusion with said coat protein in a regulated manner.