Colony hybridization is a procedure that allows the detection of cells containing nucleic acid sequences of interest (1). In this method, microbial colonies grown on, or transferred to, a supporting membrane are lysed and their nucleic adds denatured to single strands and fixed in place on the membrane. The membrane is then exposed to a similarly denatured "probe" sequence, which is identical or homologous to all or part of the target sequence, under conditions favoring reannealling. Probe sequences hybridize to complementary sequences on the membrane. Positive hybridization events are then detected by determining the presence and location of probe sequences on the membrane.