The present invention relates to a method of selectively detecting Giardia lamblia in a sample. The method makes use of at least one nucleic acid probe which is a DNA or PNA sequence which hybridizes, under appropriate conditions, to the ribosomal RNA or the ribosomal DNA of Giardia lamblia but does not hybridize to the ribosomal RNA or the ribosomal DNA of other organisms (non-Giardia lamblia organisms) which may be present in a sample.